CHAPTER 23 Echinoderms 459

Position in Animal oping elsewhere; coelom budded off unique constellation of characteristics Kingdom from the archenteron (enterocoel); found in no other phylum. Among the radial and regulative (indeterminate) more striking features shown by 1. Phylum Echinodermata (e-ki no- cleavage; and endomesoderm (meso- echinoderms are as follows: der ma-ta) (Gr. echinos, sea urchin, derm derived from or with the endo- a. A system of channels composing hedgehog, derma, skin, ata, derm) from enterocoelic pouches. the water-vascular system, characterized by) belongs to the 3. Thus echinoderms, chordates, and derived from a coelomic compart- Deuterostomia branch of the animal hemichordates are presumably ment. kingdom, the members of which are derived from a common ancestor. b. A dermal endoskeleton com- enterocoelous coelomates. The other Nevertheless, their evolutionary his- posed of calcareous ossicles. phyla traditionally assigned to this tory has taken the echinoderms to c. A hemal system, whose function group are Chaetognatha, Hemichor- the point where they are very much remains mysterious, also enclosed data, and Chordata. unlike any other animal group. in a coelomic compartment. 2. Primitively, deuterostomes have the d. Their metamorphosis, which following embryological features in changes a bilateral larva to a radial common: anus developing from or Biological Contributions adult. near the blastopore, and mouth devel- 1. There is one word that best describes echinoderms: strange. They have a

environment, while radiality is of value known, but a few are commensals. On Echinoderms to animals whose environment meets the other hand, a wide variety of other Echinoderms are marine forms and them on all sides equally. Hence, the animals make their homes in or on include sea stars (also called star- body plan of present-day echinoderms echinoderms, including parasitic or fishes), brittle stars, sea urchins, sea seems to have been derived from one commensal algae, protozoa, cteno- cucumbers, and sea lilies. They repre- that was attached to the bottom by a phores, turbellarians, cirripedes, cope- sent a bizarre group sharply distin- stalk, had radial symmetry and radiat- pods, decapods, snails, clams, poly- guished from all other animals. Their ing grooves (ambulacra) for food gath- chaetes, fish, and other echinoderms. name is derived from their external ering, and had an upward-facing oral Asteroids, or sea stars (Figure 23-1), spines or protuberances. A calcareous side. Attached forms were once plenti- are commonly found on hard, rocky endoskeleton is found in all members ful, but only about 80 species, all in surfaces, but numerous species are at of the phylum, either in the form of class Crinoidea, still survive. Oddly, home on sandy or soft bottoms. Some plates or represented by scattered tiny conditions have favored survival of species are particle feeders, but many ossicles. their free-moving descendants, al- are predators, feeding particularly on The most noticeable characteristics though they are still quite radial, and sedentary or sessile prey, since sea of echinoderms are (1) spiny endo- among them are some of the most stars themselves are relatively slow skeleton of plates, (2) water-vascular abundant marine animals. Neverthe- moving. system, (3) pedicellariae, (4) dermal less, in the exception that proves the Ophiuroids—brittle stars, or ser- branchiae, and (5) radial or bira- rule (that bilaterality is adaptive for pent stars (see Figure 23-11)—are by dial symmetry. No other group with free-moving animals), at least three far the most active echinoderms, mov- such complex organ systems has radial groups of echinoderms (sea cucum- ing by their arms rather than by tube symmetry. bers and two groups of sea urchins) feet. A few species are reported to Echinoderms are an ancient group have evolved back toward bilaterality. have swimming ability, and some bur- of animals extending back to the Cam- Echinoderms have no ability to row. They may be scavengers, brow- brian period. Despite the excellent fos- osmoregulate and thus rarely venture sers, or deposit or filter feeders. Some sil record, the origin and early evolu- into brackish waters. They occur in all are commensal in large sponges, in tion of echinoderms are still obscure. It oceans of the world and at all depths, whose water canals they may live in seems clear that they descend from from intertidal to abyssal regions. great numbers. bilateral ancestors because their larvae Often the most common animals in the Holothurians, or sea cucumbers are bilateral but become radially sym- deep ocean are echinoderms. The (see Figure 23-21), are widely preva- metrical later in development. Many most abundant species found in the lent in all seas. Many are found on zoologists believe that early echino- Philippine Trench (10,540 m) was a sea sandy or mucky bottoms, where they derms were sessile and evolved radial- cucumber. Echinoderms are virtually lie concealed. Compared with other ity as an adaptation to sessile exis- all bottom dwellers, although there are echinoderms, holothurians are greatly tence. Bilaterality is of adaptive value a few pelagic species. extended in the oral-aboral axis. They to animals that travel through their No parasitic echinoderms are are oriented with that axis more or less

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460 PART 3 The Diversity of Animal Life

Characteristics of Phylum 5. A unique water-vascular system of rounded by extensions of coelom Echinodermata coelomic origin that extends from (perihemal sinuses); main circula- the body surface as a series of tion of body fluids (coelomic fluids) 1. Body unsegmented (nonmetameric) tentacle-like projections (podia, or by peritoneal cilia tube feet) that are protracted by with radial, pentamerous symme- 10. Respiration by dermal branchiae, try; body rounded, cylindrical, or increase of fluid pressure within tube feet, respiratory tree star shaped, with five or more radiat- them; an opening to the exterior (holothuroids), and bursae (ophi- ing areas, or ambulacra, alternating (madre-porite or hydropore) uroids) with interambulacral areas usually present 11. Excretory organs absent 2. No head or brain; few specialized 6. Locomotion by tube feet, which pro- 12. Sexes separate (except a few her- sensory organs; sensory system of ject from the ambulacral areas, by maphroditic) with large gonads, sin- tactile and chemoreceptors, podia, movement of spines, or by move- gle in holothuroids but multiple in terminal tentacles, photoreceptors, ment of arms, which project from most; simple ducts, with no elabo- central disc of body and statocysts rate copulatory apparatus or sec- 3. Nervous system with circumoral ring 7. Digestive system usually complete; ondary sexual structures; fertilization and radial nerves; usually two or axial or coiled; anus absent in ophi- usually external; eggs brooded in three systems of networks located at uroids some different levels in the body, varying 8. Coelom extensive, forming the peri- 13. Development through free- in degree of development according visceral cavity and the cavity of the swimming, bilateral, larval stages to group water-vascular system; coelom of (some with direct development); 4. Endoskeleton of dermal calcare- enterocoelous type; coelomic fluid metamorphosis to radial adult or ous ossicles with spines or of cal- with amebocytes subadult form; radial cleavage and

careous spicules in dermis; covered 9. Blood-vascular system (hemal sys- regulative development by an epidermis (ciliated in most); tem) much reduced, playing little 14. Autotomy and regeneration of lost pedicellariae (in some) if any role in circulation, and sur- parts conspicuous

parallel to the substrate and lying on one side. Most are suspension or

deposit feeders. Echinoids, or sea urchins (see Fig- ure 23-16), are adapted for living on the ocean bottom and always keep their oral surface in contact with the

substratum. ―Regular‖ sea urchins pre- fer hard bottoms, but sand dollars and heart urchins (―irregular‖ urchins) are A B usually found on sand. Regular ur- chins, which are radially symmetrical, feed chiefly on algae or detritus, while irregulars, which are secondarily bilat- eral, feed on small particles. Crinoids (see Figure 23-26) stretch their arms out and up like a flower’s petals and feed on plankton and sus- pended particles. Most living species become detached from their stems as adults, but they nevertheless spend C D most of their time on the substrate, holding on by means of aboral ap- Figure 23-1 pendages called cirri. Some sea stars (class Asteroidea) from the Pacific. A, Cushion star Pteraster tesselatus can secrete incredible quantities of mucus as a defense. B, Choriaster granulatus scavenges dead animals on The zoologist who admires the shallow Pacific reefs. C, Tosia queenslandensis from the Great Barrier Reef browses encrusting fascinating structure and function organisms. D, Crossaster papposus, rose star, feeds on other sea stars. of echinoderms can share with the

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CHAPTER 23 Echinoderms 461 layperson an admiration of the beauty damages the delicate epidermal mem- shoreline where large numbers may of their symmetry, often enhanced by brane, destroying the dermal branchiae aggregate on rocks. Sometimes they bright colors. Many species are rather and ultimately the animal itself. Unfor- cling so tenaciously that they are diffi- drab, but others may be orange, red, tunately, other soft-bodied inverte- cult to dislodge without tearing off purple, blue, and often multicolored. brates are also damaged. However, the some tube feet. They also live on Because of the spiny nature of oysters remain with their shells tightly muddy or sandy bottoms and among their structure, echinoderms are not closed until the quicklime is degraded. coral reefs. They are often brightly col- often prey of other animals—except Echinoderms have been widely ored and range in size from a centime- other echinoderms (sea stars). Some used in developmental studies, for ter in greatest diameter to about a fishes have strong teeth and other their gametes are usually abundant and meter across. Asterias (Gr. asteros, a adaptations that enable them to feed easy to collect and handle in the labo- star) is one of the common genera of on echinoderms. A few mammals, ratory. Investigators can follow embry- the east coast of the United States and such as sea otters, feed on sea urchins. onic developmental stages with great is commonly studied in zoology labo- In scattered parts of the world, humans accuracy. We know more about the ratories. Pisaster (Gr. pisos, a pea, relish sea urchin gonads, either raw or molecular biology of sea urchin devel- asteros, a star) is common on the west roasted on the half shell. Trepang, the opment than that of almost any other coast of the United States, as is Der- cooked, protein-rich body wall of cer- embryonic system. Artificial partheno- masterias (Gr. dermatos, skin, leather, tain large sea cucumbers, is a delicacy genesis was first discovered in sea asteros, a star), the leather star. in many east Asian countries. Unfortu- urchin eggs, when it was found that, nately, the intense, often illegal, fishery by treating eggs with hypertonic sea- Form and Function for sea cucumbers has severely de- water or subjecting them to a variety of External Features pleted their populations in many areas other stimuli, development would pro- of the tropical world. ceed without sperm. Sea stars are composed of a central Sea stars feed on a variety of mol- disc that merges gradually with the luscs, crustaceans, and other inverte- tapering arms (rays). The body is brates. In some areas they may per- Class Asteroidea somewhat flattened, flexible, and cov- form an important ecological role as a Sea stars, often called starfishes, dem- ered with a ciliated, pigmented epider- top carnivore in the habitat. Their chief onstrate the basic features of echino- mis. The mouth is centered on the economic impact is on clams and oys- derm structure and function very well, under, or oral, side, surrounded by a ters. A single starfish may eat as many and they are easily obtainable. Thus soft peristomial membrane. An ambu- as a dozen oysters or clams in a day. we will consider them first, then com- lacrum (pl., ambulacra, L. ambu- To rid shellfish beds of these pests, ment on major differences shown by lacrum, a covered way, an alley, a quicklime is sometimes spread over other groups. walk planted with trees) or ambu- areas where they abound. Quicklime Sea stars are familiar along the lacral area, runs from the mouth on

Arms

A B Spines Central disc

Mouth

Madreporite Tube feet Arm Ambulacral grooves Anus Sensory tentacles

Figure 23-2 External anatomy of asteroid. A, Aboral view. B, Oral view.

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462 PART 3 The Diversity of Animal Life

Tube feet Water vascular system

A

Pyloric duct Madreporite Ambulacral ridge

Anus Gonad Ampulla Eye spot Cardiac stomach Pyloric stomach B Intestinal cecum Pyloric cecum Polian vesicle

C Spine Madreporite Apical longitudinal Stone canal Ring canal muscle and nerve Papula

Ossicle

Pyloric cecum

Tiedemann's Lateral canal Gonad body Radial canal Ampulla Ampulla Podium Tube foot Radial Sucker canal Radial nerve Figure 23-3 A, Internal anatomy of a sea star. B, Water-vascular system. Podia penetrate between ossicles. (Polian vesicles are not present in Asterias.) C, Cross section of arm at level of gonads, illustrating open ambulacral grooves. the oral side of each arm to the tip of lacral groove (Figure 23-3C), between crinoids are said to be open, and those the arm. Sea stars typically have five the rows of tube feet. The nerve is very of the other groups are closed. arms, but they may have more (Fig- superficially located, covered only by The aboral surface is usually rough ure 23-1D), and there are as many thin epidermis. Under the nerve is an and spiny, although spines of many ambulacral areas as there are arms. An extension of the coelom and the radial species are flattened, so that the sur- ambulacral groove is found along canal of the water-vascular system, all face appears smooth (Figure 23-1C). the middle of each ambulacral area, of which are external to the underlying Around the bases of spines are groups and the groove is bordered by rows of ossicles (Figure 23-3C). In all other of minute, pincerlike pedicellariae, tube feet (podia) (Figure 23-2). These classes of living echinoderms except bearing tiny jaws manipulated by mus- in turn are usually protected by mov- crinoids, these structures are covered cles (Figure 23-4). These jaws help able spines. A large radial nerve can by ossicles or other dermal tissue; thus keep the body surface free of debris, be seen in the center of each ambu- ambulacral grooves in asteroids and protect the papulae, and sometimes

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CHAPTER 23 Echinoderms 463

Spine

A B C D E

Figure 23-4 Pedicellariae of sea stars and sea urchins. A, Forceps-type pedicellaria of Asterias. B, and C, Scissors-type pedicellariae of Asterias; size relative to spine is shown in B. D, Tridactyl pedicellaria of Strongylocentrotus. E, Globiferous pedicellaria of Strongylocentrotus. F, Close-up view of the aboral surface of the sea star Pycnopodia helianthoides. Note the large pedicellariae, as well as groups of small pedicellariae around the spines. Many thin- walled papulae can be seen.

F aid in food capture. Papulae (dermal tubercles that make up the spiny sur- and projects into the papulae. The cili- branchiae or skin gills) are soft deli- face. Ossicles are penetrated by a ated peritoneal lining of the coelom cate projections of the coelomic cavity, meshwork of spaces, usually filled circulates the fluid around the body covered only with epidermis and lined with fibers and dermal cells. This inter- cavity and into the papulae. Exchange internally with peritoneum; they ex- nal meshwork structure is described as of respiratory gases and excretion of tend out through spaces between ossi- stereom and is unique to echino- nitrogenous waste, principally ammo- cles and are concerned with respira- derms. nia, take place by diffusion through tion (Figures 23-3C, and 23-4F). Also Muscles in the body wall move the the thin walls of papulae and tube feet. on the aboral side are the inconspicu- rays and can partially close the ambu- Some wastes may be picked up by ous anus and the circular mad- lacral grooves by drawing their mar- coelomocytes, which migrate through reporite (Figure 23-2A), a calcareous gins together. the epithelium of the papulae or tube sieve leading to the water-vascular feet to the exterior, or the tips of papu- system. Coelom, Excretion, and lae containing waste-laden coelomo- cytes may pinch off. Respiration Endoskeleton The coelomic compartments of larval Beneath the epidermis of sea stars is a echinoderms give rise to several struc- Water-Vascular System mesodermal endoskeleton of small cal- tures in adults, one of which is a spa- The water-vascular system is another careous plates, or ossicles, bound cious body coelom filled with fluid. coelomic compartment and is unique together with connective tissue. From The fluid contains amebocytes (coelo- to echinoderms. Showing exploitation these ossicles project the spines and mocytes), bathes the internal organs, of hydraulic mechanisms to a greater

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464 PART 3 The Diversity of Animal Life degree than in any other animal group, it is a system of canals and specialized tube feet that, together with the dermal ossicles, has determined the evolution- ary potential and limitations of this phylum. In sea stars the primary func- tions of the water-vascular system are locomotion and food gathering, in addition to respiration and excretion. Structurally, the water-vascular sys- tem opens to the outside through small pores in the madreporite. The mad- reporite of asteroids is on the aboral surface (Figure 23-2A) and leads into the stone canal, which descends A B toward the ring canal around the Figure 23-5 mouth (Figure 23-3B). Radial canals diverge from the ring canal, one into A, Orthasterias koehleri eating a clam. B, This Pycnopodia helianthoides has been overturned while eating a large sea urchin Strongylocentrotus franciscanus. This sea star has 20 to 24 arms and can the ambulacral groove of each ray. range up to 1 m in diameter (arm tip to arm tip). Also attached to the ring canal are four or five pairs of folded, pouchlike Tiedemann’s bodies and from one to five polian vesicles (polian vesicles tube foot can raise the middle of the star may progress. If the radial nerve in are absent in some sea stars, such as disclike end, creating suction when the an arm is cut, podia in that arm lose Asterias). Tiedemann’s bodies may pro- end is applied to a firm substrate. It has coordination, although they can still duce coelomocytes, and polian vesicles been estimated that by combining function. If the circumoral nerve ring is are apparently for fluid storage. mucous adhesion with suction, a single cut, podia in all arms become uncoor- A series of small lateral canals, podium can exert a pull equal to 25 to dinated, and movement ceases. each with a one-way valve, connects the 30 g. Coordinated action of all or many radial canal to the cylindrical podia, or of the tube feet is sufficient to draw the tube feet, along the sides of the ambu- animal up a vertical surface or over Feeding and Digestive System lacral groove in each ray. Each podium rocks. The ability to move while firmly The mouth on the oral side leads is a hollow, muscular tube, the inner adhering to the substrate is a clear through a short esophagus to a large end of which is a muscular sac, or advantage to an animal living in a stomach in the central disc. The lower ampulla, that lies within the body sometimes wave-churned environment. (cardiac) part of the stomach can be coelom (Figure 23-3A and C), and the On a soft surface, such as muck or everted through the mouth during outer end of which usually bears a sand, suckers are ineffective (numer- feeding (Figure 23-2B), and excessive sucker. Some species lack suckers. ous sand-dwelling species have no eversion is prevented by gastric liga- Podia pass to the outside between ossi- suckers), so the tube feet are em- ments. The upper (pyloric) part is cles in the ambulacral groove. ployed as legs. Locomotion becomes smaller and connects by ducts to a pair The water-vascular system operates mainly a stepping process. Most sea of large pyloric ceca (digestive hydraulically and is an effective loco- stars can move only a few centimeters glands) in each arm (Figure 23-3A). motor mechanism. The valves in the lat- per minute, but some very active ones Digestion is mostly extracellular, eral canals prevent backflow of fluid can move 75 to 100 cm per minute; for although some intracellular digestion into the radial canals. Each tube foot example, Pycnopodia (Gr. pyknos, may occur in the ceca. A short intestine has in its walls connective tissue that compact, dense, pous, podos, foot) leads aborally from the pyloric stom- maintains the cylinder at a relatively (Figure 23-5B). When inverted, a sea ach, and there are usually a few small, constant diameter. Contraction of mus- star bends its rays until some of the saclike intestinal ceca (Figure 23-3A). cles in the ampulla forces fluid into the tubes reach the substratum and attach The anus is inconspicuous, and some podium, extending it. Conversely, con- as an anchor; then it slowly rolls over. sea stars lack an intestine and anus. traction of the longitudinal muscles in Tube feet are innervated by the Many sea stars are carnivorous the tube foot retracts the podium, forc- central nervous system (ectoneural and and feed on molluscs, crustaceans, ing fluid back into the ampulla. Con- hyponeural systems, see following polychaetes, echinoderms, other in- traction of muscles in one side of the text). Nervous coordination enables vertebrates, and sometimes small fish. podium bends the organ toward that tube feet to move in a single direction, Sea stars consume a wide range of side. Small muscles at the end of the although not in unison, so that the sea food items, but many show particular

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CHAPTER 23 Echinoderms 465

Anus Madreporite

Aboral Dorsal hemal sac ring Stone canal Pyloric hemal Axial canal organ Axial Cardiac Gastric stomach hemal sinus ring

Ring canal Perihemal ring

Mouth Radial hemal canal Oral hemal ring

Figure 23-7 Hemal system of asteroids. The main perihemal channel is the thin-walled axial sinus, which encloses both the axial organ and the stone canal. Other features of the hemal system are shown. Figure 23-6 Crown-of-thorns star Acanthaster planci feeding on coral. Puncture wounds from its spines are painful; the spines are equipped with poison Some asteroids feed heavily on function in all echinoderms is unclear. glands. The hemal system has little or nothing molluscs (Figure 23-5A), and Asterias is a significant predator on commer- to do with circulation of body fluids. It cially important clams and oysters. is a system of tissue strands enclosing preferences (Figures 23-5 and 23-6). When feeding on a bivalve, a sea star unlined sinuses and is itself enclosed Some select brittle stars, sea urchins, or will hump over its prey, attaching its in another coelomic compartment, the sand dollars, swallowing them whole podia to the valves, and then exert a perihemal channels (Figure 23-7). and later regurgitating undigestible steady pull, using its feet in relays. A The hemal system may be useful in ossicles and spines (Figure 23-5B). force of some 1300 g can thus be distributing digested products, but Some attack other sea stars, and if they exerted. In half an hour or so the its specific functions are not really are small compared with their prey, adductor muscles of the bivalve fatigue known. they may attack and begin eating at the and relax. With a very small gap avail- end of one arm. able, the star inserts its soft everted Nervous System stomach into the space between the valves and wraps it around the soft The nervous system consists of three Since 1963 there have been numerous units at different levels in the disc and reports of increasing numbers of the parts of the shellfish. After feeding, the arms. Chief of these systems is the oral crown-of-thorns starfish (Acanthaster sea star draws its stomach inward by planci [Gr. akantha, thorn, asteros, contraction of the stomach muscles (ectoneural) system composed of a star]) (Figure 23-6) that were damaging and relaxation of body-wall muscles. nerve ring around the mouth and a large areas of coral reef in the Pacific Ocean. Some sea stars feed on small parti- main radial nerve into each arm. It Crown-of-thorns stars feed on coral polyps, cles, either entirely or in addition to car- appears to coordinate the tube feet. A and they sometimes occur in large aggrega- nivorous feeding. Plankton and other deep (hyponeural) system lies aboral tions, or ―herds.‖ There is some evidence that organic particles coming in contact with to the oral system, and an aboral sys- outbreaks have occurred in the past, but an the animal’s surface are carried by the tem consists of a ring around the anus increase in frequency during the past 30 epidermal cilia to the ambulacral and radial nerves along the roof of years suggests that some human activity each ray. An epidermal nerve plexus may be affecting the starfish. Efforts to con- grooves and then to the mouth. or nerve net freely connects these trol the organism are very expensive and of systems with the body wall and related questionable effectiveness.The controversy Hemal System continues, especially in Australia, where it is structures. The epidermal plexus coor- exacerbated by extensive media coverage. The so-called hemal system is not very dinates responses of the dermal well developed in asteroids, and its branchiae to tactile stimulation—the

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466 PART 3 The Diversity of Animal Life only instance known in echinoderms face, and the larval right side becomes in which coordination occurs through the aboral surface (Figure 23-9). Corre- a nerve net. spondingly, the larval mouth and anus Sense organs are not well devel- disappear, and a new mouth and anus oped. Tactile organs and other sensory form on what were originally the left cells are scattered over the surface, and and right sides, respectively. The por- an ocellus is at the tip of each arm. tion of the anterior coelomic compart- Their reactions are mainly to touch, ment from the left side expands to temperature, chemicals, and differ- form the ring canal of the water- ences in light intensity. Sea stars are vascular system around the mouth, and usually more active at night. then it grows branches to form the radial canals. As the short, stubby arms and the first podia appear, the animal Reproductive System, detaches from its stalk and begins life Regeneration, and Autotomy as a young sea star. Figure 23-8 Most sea stars have separate sexes. A Pacific sea star Echinaster luzonicus can pair of gonads lies in each interradial reproduce itself by splitting across the disc, then space (Figure 23-3A). Fertilization is regenerating missing arms. The one shown here Class Ophiuroidea has evidently regenerated six arms from the external and occurs in early summer longer one at top left. Brittle stars are largest of the major when eggs and sperm are shed into groups of echinoderms in numbers of the water. A secretion from neurose- species, and they are probably the cretory cells located on the radial Figures 8-7A and 8-11A). Gastrulation most abundant also. They abound in nerves stimulates maturation and shed- is by invagination, and the anterior end all types of benthic marine habitats, ding of asteroid eggs. of the archenteron pinches off to even carpeting the abyssal sea bottom Echinoderms can regenerate lost become the coelomic cavity, which in many areas. parts. Sea star arms can regenerate expands in a U shape to fill the blasto- readily, even if all are lost. Sea stars coel. Each of the legs of the U, at the also have the power of autotomy and posterior, constricts to become a sepa- Form and Function can cast off an injured arm near the rate vesicle, and these eventually give Apart from the typical possession of base. Regeneration of a new arm may rise to the main coelomic compart- five arms, brittle stars are surprisingly take several months. ments of the body (metacoels, called different from asteroids. The arms of Some species can regenerate a somatocoels in echinoderms). The brittle stars are slender and sharply set complete new sea star (Figure 23-8) anterior portion of the U undergoes off from the central disc (Figure 23-11). from a detached arm that contains a subdivision to form the protocoels and They have no pedicellariae or papulae, part (about one-fifth) of the central mesocoels (called axocoels and hydro- and their ambulacral grooves are disc. In former times fishermen used to coels in echinoderms) (Figure 23-9). closed and covered with arm ossicles. dispatch sea stars they collected from The left hydrocoel will become the Their tube feet are without suckers; their oyster beds by chopping them in water-vascular system, and the left they aid in feeding but are of limited half with a hatchet—a worse than axocoel will give rise to the stone canal use in locomotion. In contrast to aster- futile activity. Some sea stars repro- and perihemal channels. The right axo- oids, the madreporite of ophiuroids is duce asexually under normal condi- coel and hydrocoel will disappear. The located on the oral surface, on one of tions by cleaving the central disc, each free-swimming larva has cilia arranged the oral shield ossicles (Figure 23-12). part regenerating the rest of the disc in bands and is called a bipinnaria Ampullae on the podia are absent, and and missing arms. (Figure 23-10A). These ciliated tracts force for protrusion of the podium is become extended into larval arms. generated by a proximal muscular por- Soon the larva grows three adhesive tion of the podium. Development arms and a sucker at its anterior end Each jointed arm consists of a In some species the liberated eggs are and is then called a brachiolaria. At column of articulated ossicles (the so- brooded, either under the oral side of that time it attaches to the substratum, called vertebrae), connected by mus- the animal or in specialized aboral forms a temporary attachment stalk, cles and covered by plates. Locomo- structures, and development is direct, and undergoes metamorphosis. tion is by arm movement. Arms are but in most species embryonating eggs Metamorphosis involves a dra- moved forward in pairs and are placed are free in the water and hatch to free- matic reorganization of a bilateral larva against the substratum, while one (any swimming larvae. into a radial juvenile. The anteroposte- one) is extended forward or trailed Early embryogenesis shows a typi- rior axis of the larva is lost, and what behind, and the animal is pulled or cal primitive deuterostome pattern (see was the left side becomes the oral sur- pushed along in a jerky fashion.

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CHAPTER 23 Echinoderms 467

Original anus (disappears) Hydrocoel Hydrocoel

Left somatocoel Right Formative somatocoel arms Original mouth (disappears) New mouth (left side) Left hydrocoel Mouth (oral surface) Left axocoel New anus Stalk Anus

Figure 23-9 Asteroid metamorphosis. The left somatocoel becomes the oral coelom, and the right somatocoel becomes the aboral coelom. The left hydrocoel becomes the water-vascular system and the left axocoel the stone canal and perihemal channels. The right axocoel and hydrocoel are lost.

Five movable plates that serve as jaws surround the mouth (Figure 23-12). There is no anus. The skin is leathery, with dermal plates and spines arranged in characteristic patterns. Surface cilia are mostly lacking.

A B C The visceral organs are confined to the central disc, since the rays are too Bipinnaria Brachiolaria Ophiopluteus slender to contain them (Figure 23-13). The stomach is saclike, and there is no intestine. Indigestible material is cast out of the mouth. Five pairs of invaginations called bursae open toward the oral surface by genital slits at the bases of the arms. Water circulates in and out of these sacs D E F for exchange of gases. On the coelomic Echinopluteus Auricularia Doliolaria wall of each bursa are small gonads that Figure 23-10 discharge into the bursa their ripe sex Larvae of echinoderms. A, Bipinnaria of asteroids. B, Brachiolaria of asteroids. C, Ophiopluteus of cells, which pass through the genital ophiuroids. D, Echinopluteus of echinoids. E, Auricularia of holothuroids. F, Doliolaria of crinoids. slits into the water for fertilization (Fig- ure 23-14A). Sexes are usually separate;

a few ophiuroids are hermaphroditic. Some brood their young in the bursae; the young escape through the genital slits or by rupturing the aboral disc. The larva is called an ophiopluteus, and its

ciliated bands extend onto delicate, beautiful larval arms (Figure 23-10C). During metamorphosis to a juvenile, there is no temporarily attached phase, as there is in asteroids.

Water-vascular, nervous, and hemal systems are similar to those of A sea stars. Each arm contains a small coelom, a radial nerve, and a radial Figure 23-11 canal of the water-vascular system. A, Brittle star Ophiura lutkeni (class Ophiuroidea). Brittle stars do not use their tube feet for locomotion but can move rapidly (for an Biology echinoderm) by means of their arms. B, Basket Brittle stars tend to be secretive, living star Astrophyton muricatum (class Ophiuroidea). Basket stars extend their many-branched arms on hard bottoms where little or no to filter feed, usually at night. light penetrates. They are generally B

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468 PART 3 The Diversity of Animal Life

Bursal slits

Mouth

Madreporite

Spines A

Podial pores

Jaw Oral shield Oral arm shields

Figure 23-12 Oral view of spiny brittle star Ophiothrix.

sion feeding. Podia are important in transferring food to the mouth. Some Bursae Arm spines brittle stars extend arms into the water B and catch suspended particles in Figure 23-14 mucous strands between arm spines. A, This brittle star Ophiopholis aculeata has its Regeneration and autotomy are bursae swollen with eggs, which it is ready to even more pronounced in brittle stars expel. The arms have been broken and are regenerating. B, Oral view of a basket star than in sea stars. Many seem very frag- Gorgonocephalus eucnemis, showing Gonads Stomach ile, releasing an arm or even part of pouches pentaradial symmetry. the disc at the slightest provocation. Some can reproduce asexually by cleaving the disc; each progeny then Arm shield regenerates the missing parts. Some common ophiuroids along the coast of the United States are Figure 23-13 Amphipholis (Gr. amphi, both sides of, Ophiuroid with aboral disc wall cut away to show principal internal structures. The bursae pholis, horny scale) (viviparous and are fluid-filled sacs in which water constantly hermaphroditic), Ophioderma (Gr. ophis, circulates for respiration. They also serve as snake, dermatos, skin), Ophiothrix brood chambers. Only bases of arms are (Gr. ophis, snake, thrix, hair), and shown. Ophiura (Gr. ophis, snake, oura, tail) (Figure 23-11). The basket stars Figure 23-15 Purple sea urchin Strongylocentrotus purpuratus Gorgonocephalus (Gr. Gorgo, name of is common along the Pacific coast of North negatively phototropic and insinuate a female monster of terrible aspect, America where there is heavy wave action. themselves into small crevices between kephalƒ, a head) (Figure 23-14B) rocks, becoming more active at night. and Astrophyton (Gr. asteros, star, They are commonly fully exposed on phyton, creature, animal) (Figure Class Echinoidea the bottom in the permanent darkness 23-11B) have arms that branch repeat- of the deep sea. Ophiuroids feed on a edly. Most ophiuroids are drab, but Echinoids have a compact body en- variety of small particles, either brows- some are attractive, with bright color closed in an endoskeletal test, or shell. ing food from the bottom or suspen- patterns (Figure 23-14A). Dermal ossicles, which have become

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CHAPTER 23 Echinoderms 469

orders to which they belong have become secondarily bilateral; their spines are usually very short. Regular urchins move by means of their tube feet, with some assistance from their spines, and irregular urchins move chiefly by their spines (Figure 23-17). Some echinoids are quite colorful. Echinoids have a wide distribution in all seas, from intertidal regions to A B deep oceans. Regular urchins often prefer rocky or hard bottoms, whereas sand dollars and heart urchins like to burrow into a sandy substrate. Distrib- uted along one or both coasts of North America are common genera of regular urchins (Arbacia [Gr. Arbakƒs, first king of Media], Strongylocentrotus [Gr. strongylos, round, compact, ken- tron, point, spine] [Figure 23-15], and Lytechinus [Gr. lytos, dissolvable, bro- C D ken, echinos, sea urchin]) and sand dollars (Dendraster [Gr. dendron, tree, stick, asteros, star] and Echinarach- nius [Gr. echinos, sea urchin, arachnƒ, spider]). The West Indies- Florida region is rich in echinoderms, including echinoids, of which Dia- dema (Gr. diade|, to bind around), with its long, needle-sharp spines, is a notable example (Figure 23-16D).

Form and Function The echinoid test is a compact skeleton of 10 double rows of plates that bear E movable, stiff spines (Figure 23-19). The Figure 23-16 plates are sutured firmly. The five pairs Diversity among regular sea urchins (class Echinoidea). A, Pencil urchin Eucidaris tribuloides. Members of ambulacral rows are homologous to of this order have many primitive characters and have survived since the Paleozoic era. They may be the five arms of sea stars and have pores closest in resemblance to the common ancestor of all other extant echinoids. B, Slate-pencil urchin (Figure 23-19B) through which long Heterocentrotus mammilatus. The large, triangular spines of this urchin were formerly used for writing tube feet extend. The plates bear small on slates. C, Aboral spines of the intertidal urchin Colobocentrotus atratus are flattened and tubercles on which the round ends of mushroom shaped, while the marginal spines are wedge shaped, giving the animal a streamlined form to withstand pounding surf. D, Diadema antillarum is a common species in the West Indies and the spines articulate as ball-and-socket Florida. E, Astropyga magnifica is one of the most spectacularly colored sea urchins, with bright-blue joints. Spines are moved by small mus- spots along its interambulacral areas. cles around the bases. There are several kinds of pedi- cellariae, the most common of which closely fitting plates, make up the test. areas extend up to the area around the are three jawed and are mounted on Echinoids lack arms, but their tests anus (periproct). The majority of liv- long stalks (Figure 23-4D and E). Pedi- reflect the typical pentamerous plan of ing species of sea urchins are ―regular‖; cellariae help keep the body clean, echinoderms in their five ambulacral they have a hemispherical shape, radial especially by preventing marine larvae areas. The most notable modification symmetry, and medium to long spines from settling on the body surface. of the ancestral body plan is that the (Figures 23-15 and 23-16). Sand dollars Pedicellariae of many species bear poi- oral surface has expanded around to (Figure 23-17) and heart urchins (Fig- son glands, and the toxin paralyzes the aboral side, so that the ambulacral ure 23-18) are ―irregular‖ because the small prey.

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470 PART 3 The Diversity of Animal Life

Inside the test (Figure 23-19) are the coiled digestive system and a complex chewing mechanism (in reg- ular urchins and in sand dollars), called Aristotle’s lanter n (Fig- ure 23-20), to which the teeth are attached. A ciliated siphon connects the esophagus to the intestine and enables water to bypass the stomach to concentrate food for digestion in the intestine. Sea urchins eat algae and other organic material, which A B they graze with their teeth. Sand dol- lars have short club-shaped spines Figure 23-17 that move the sand and its organic Two sand dollar species. A, Encope grandis as normally found burrowing near the surface on a sandy contents over the aboral surface and bottom. B, Removed from the sand. The short spines and petaloids on the aboral surface of this Encope micropora are easily seen. down the sides. Fine food particles drop between the spines, and ciliated tracts on the oral side carry the parti- cles to the mouth. Hemal and nervous systems are basically similar to those of asteroids. Periproct Ambulacral grooves are closed, and Aboral pole radial canals of the water-vascular sys- tem run just beneath the test, one in each of the ambulacral radii (Fig-

ure 23-19). Ampullae for the podia are within the test, and each ampulla usu- Mouth ally communicates with its podium by two canals through pores in the ambu- lacral plate; consequently, such pores A B in the plates are in pairs. Peristomial Figure 23-18 gills, where present, are of little or no An irregular echinoid Meoma, one of the largest heart urchins (test up to 18 cm). Meoma occurs in importance in respiratory gas ex- the West Indies and from the Gulf of California to the Galápagos Islands. A, Aboral view. Anterior change, this function being carried ambulacral area is not modified as a petaloid in the heart urchins, although it is in the sand dollars. out principally by the other podia. In B, Oral view. Note curved mouth at anterior end and periproct at posterior end. irregular urchins respiratory podia are thin walled, flattened, or lobulate and Five converging teeth surround the Diadema antillarum is not nearly as are arranged in ambulacral fields prominent as it once was. In January 1983, mouth of regular urchins. In some sea called petaloids on the aboral sur- an epidemic swept through the Caribbean urchins branched gills (modified podia) face. Irregular urchins also have short, and along the Florida Keys. Its cause has encircle the peristome. The anus, geni- suckered, single-pored podia in the never been determined, but it decimated tal pores, and madreporite are located ambulacral and sometimes interambu- the Diadema population, leaving less than aborally in the periproct region (Fig- lacral areas; these podia function in 5% of the original numbers. Other species ure 23-19). Sand dollars also have teeth, food handling. of sea urchins were unaffected. However, and the mouth is located at about the Sexes are separate, and both eggs various types of algae, formerly grazed center of the oral side, but the anus has heavily by the Diadema have increased and sperm are shed into the sea for shifted to the posterior margin or even external fertilization. Some, such as greatly on the reefs, and Diadema popula- the oral side of the disc, so that an tions have not recovered. This abundance certain pencil urchins, brood their anteroposterior axis and bilateral sym- of algae has had a disastrous effect on coral young in depressions between the reefs around Jamaica. Herbivorous fish metry can be recognized. Bilateral sym- spines. Echinopluteus larvae (Fig- around that island had been chronically metry is even more accentuated in heart ure 23-10D) of nonbrooding echi- overharvested, and then, after the Diadema urchins, with the anus near the poste- noids may live a planktonic existence epidemic, there was nothing left to control rior on the oral side and the mouth for several months and then meta- algal overgrowth. Coral reefs around moved away from the oral pole toward morphose quickly into young Jamaica have been largely destroyed. the anterior (Figure 23-18). urchins.

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CHAPTER 23 Echinoderms 471

Genital pore Stone canal Periproct Gonad Madreporite Intestine Anus Axial organ

A Esophagus B Siphon Stomach Polian Primary tubercle vesicle Spine

Radial Test Ampullae canal Ring Tube canal Nerve Radial Pedicellarium feet Mouth ring nerve cord Pores for Aristotle's tube feet

Figure 23-19 A, Internal structure of a sea urchin; water-vascular system in tan. B, Detail of portion of endoskeleton.

joining together). Along the Pacific Esophagus coast there are several species of Cucumaria (Figure 23-21C) and the Alveolus striking reddish brown Parastichopus Protractor muscle (Gr. para, beside, stichos, line or row, pous, podos, foot) (Figure 23-21A), with very large papillae.

Form and Function Auricle Test Mouth Retractor muscle The body wall is usually leathery, with Teeth tiny ossicles embedded in it (Fig- Figure 23-20 ure 23-22), although a few species Aristotle’s lantern, a complex mechanism used by the sea urchin for masticating its food. Five pairs of retractor muscles draw the lantern and teeth up into the test; five pairs of protractors push the lantern have large ossicles forming a dermal down and expose the teeth. Other muscles produce a variety of movements. Only major skeletal armor (Figure 23-21B). Because of the parts and muscles are shown in this diagram. elongate body form of sea cucumbers, they characteristically lie on one side. In some species locomotor tube feet ossicles are much reduced in most, so are equally distributed to the five Class Holothuroidea that the animals are soft bodied. Some ambulacral areas (Figure 23-21C) or In a phylum characterized by odd ani- species crawl on the surface of the sea all over the body, but most have well- mals, class Holothuroidea (sea cucum- bottom, others are found beneath developed tube feet only in the ambu- bers) contains members that both rocks, and some are burrowers. lacra normally applied to the substra- structurally and physiologically are Common species along the east tum (Figure 23-21A and B). Thus a among the strangest. These animals coast of North America are Cucumaria secondary bilaterality is present, albeit have a remarkable resemblance to the frondosa (L. cucumis, cucumber), Scle- of quite different origin from that of vegetable after which they are named rodactyla briareus (Gr. skleros, hard, irregular urchins. The side applied to (Figure 23-21). Compared with other daktylos, finger) (Figure 23-23), and the substratum has three ambulacra echinoderms, holothurians are greatly the translucent, burrowing Leptosy- and is called the sole; tube feet in the elongated in the oral-aboral axis, and napta (Gr. leptos, slender, synapsis, dorsal ambulacral areas, if present, are

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472 PART 3 The Diversity of Animal Life

A B C Figure 23-21 Sea cucumbers (class Holothuroidea). A, Common along the Pacific coast of North America, Parastichopus californicus grows to 50 cm in length. Its tube feet on the dorsal side are reduced to papillae and warts. B, In sharp contrast to most sea cucumbers, the surface ossicles of Psolus chitonoides are developed into a platelike armor. The ventral surface is a flat, soft, creeping sole, and the mouth (surrounded by tentacles) and anus are turned dorsally. C, Tube feet are found in all ambulacral areas of Cucumaria miniata but are better developed on its ventral side, shown here.

ure 23-23). A respiratory tree com- posed of two long, many-branched tubes also empties into the cloaca, which pumps seawater into it. The res- piratory tree serves for both respiration and excretion and is not present in any other group of living echinoderms. Gas exchange also occurs through the skin and tube feet. The hemal system is more well developed in holothurians than in other echinoderms. The water-vascular system is peculiar in that the mad- reporite lies free in the coelom. Sexes are usually separate, but some holothurians are hermaphroditic. Among echinoderms, only sea cucumbers have a single gonad, and this is considered a primitive character. The gonad is usually in the form of one or two clusters of tubules that join at the gonoduct. Figure 23-22 Fertilization is external, and the free- Ossicles of sea cucumbers are usually microscopic bodies buried in the leathery dermis. They can be swimming larva is called an auricularia extracted from the tissue with commercial bleach and are important taxonomic characteristics. The ossicles shown here, called tables, buttons, and plates, are from the sea cucumber Holothuria difficilis. (Figure 23-10E). Some species brood the They illustrate the meshwork (stereom) structure observed in ossicles of all echinoderms at some young either inside the body or some- stage in their development ( 250). where on the body surface.

usually without suckers and may be The coelomic cavity is spacious Biology modified as sensory papillae. All tube and fluid filled and has many coelomo- Sea cucumbers are sluggish, moving feet, except oral tentacles, may be cytes. Because of the reduction in der- partly by means of their ventral tube absent in burrowing forms. mal ossicles, they no longer function feet and partly by waves of contraction The oral tentacles are 10 to 30 as an endoskeleton, and the fluid-filled in the muscular body wall. More retractile, modified tube feet around coelom now serves as a hydrostatic sedentary species trap suspended food the mouth. The body wall contains cir- skeleton. particles in the mucus of their cular and longitudinal muscles along The digestive system empties pos- outstretched oral tentacles or pick up the ambulacra. teriorly into a muscular cloaca (Fig- particles from the surrounding bottom.

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CHAPTER 23 Echinoderms 473

can cast out a part of their viscera by a Tentacles strong muscular contraction that may either rupture the body wall or evert its Aquapharyngeal bulb contents through the anus. Lost parts Pharynx Ring canal are soon regenerated. Certain species Polian vesicle have organs of Cuvier (cuvierian tu- Madreporite Gonad bules), which are attached to the pos- Stomach Ventral terior part of the respiratory tree and Dorsal hemal hemal sinus can be expelled in the direction of an sinus Intestine enemy (Figure 23-24C). These tubules Transverse sinus become long and sticky after expul- Tube feet sion, and some contain toxins. Body wall B Longitudinal muscle There is an interesting commensal bands relationship between some sea cucum- Respiratory tree Cloacal muscles bers and a small fish, Carapus, that

Cloaca uses the cloaca and respiratory tree of the sea cucumber as shelter. A Anus Figure 23-23 Anatomy of the sea cucumber Sclerodactyla. A, Internal. B, External. Red, hemal system. Class Crinoidea Crinoids include sea lilies and feather stars. They have several primitive char- acters. As fossil records reveal, crinoids were once far more numerous than they are now. They differ from other echinoderms by being attached during a substantial part of their lives. Sea lilies have a flower-shaped body that is placed at the tip of an attached stalk (Figure 23-25). Feather stars have long, many-branched arms, and adults are free moving, though they may remain in the same spot for long periods (Fig- ure 23-26). During metamorphosis A B feather stars become sessile and

stalked, but after several months they detach and become free moving. Many crinoids are deep-water forms, but feather stars may inhabit shallow waters, especially in the Indo-Pacific

Figure 23-24 and West-Indian–Caribbean regions, A, Eupentacta quinquesemita extends its where the largest numbers of species tentacles to collect particulate matter in the are found. water, then puts them one by one into its mouth and cleans the food from them. B, Moplike tentacles of Parastichopus californicus are used Form and Function for deposit feeding on the bottom. C, Bohadschia argus expels its cuvierian tubules, modified parts The body disc, or calyx, is covered of its respiratory tree, when it is disturbed. These with a leathery skin (tegmen) contain- sticky strands, containing a toxin, discourage ing calcareous plates. The epidermis is C potential predators. poorly developed. Five flexible arms branch to form many more arms, each They then stuff their tentacles into the Sea cucumbers have a peculiar with many lateral pinnules arranged pharynx, one by one, sucking off the power of what appears to be self-muti- like barbs on a feather (Figure 23-25). food material (Figure 23-24A). Others lation but may be a mode of defense. Calyx and arms together are called the crawl along, grazing the bottom with When irritated or when subjected to crown. Sessile forms have a long, their tentacles (Figure 23-24B). unfavorable conditions, many species jointed stalk attached to the aboral

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474 PART 3 The Diversity of Animal Life

Pinnules

Arm

Calyx Interambulacral area

Ambulacral grooves

Figure 23-26 Comantheria briareus are crinoids found on Cirri Pacific coral reefs. They extend their arms into the water to catch food particles both during the day and at night. Stalk Mouth Arms Skeletal plates called sea daisies, they are the most recently described (1986) class of Anus echinoderms, and only two species are known so far. They are pentaradial in A B Pinnules symmetry but have no arms. Their tube feet are located around the periphery of the disc, rather than along Figure 23-25 ambulacral areas, as in other echino- Crinoid structure. A, Sea lily (stalked crinoid) with portion of stalk. Modern crinoid stalks rarely exceed derms. Their water-vascular system 60 cm, but fossil forms were as much as 20 m long. B, Oral view of calyx of the crinoid Antedon, includes two concentric ring canals; showing direction of ciliary food currents. Ambulacral grooves with podia extend from mouth along arms and branching pinnules. Food particles touching podia are tossed into ambulacral grooves and the outer ring may represent radial carried, tangled in mucus, by strong ciliary currents toward mouth. Particles falling on interambulacral canals since podia arise from it. A areas are carried by cilia first toward mouth and then outward and finally dropped off the edge, thus hydropore, homologous to the madre- keeping the oral disc clean. porite, connects the inner ring canal to the aboral surface. One species has no side of the body. This stalk is com- developed in crinoids than in most digestive tract; its oral surface is cov- posed of plates, appears jointed, and other echinoderms. Sense organs are ered by a membranous velum, by may bear cirri. Madreporite, spines, scanty and primitive. which it apparently absorbs nutrients. and pedicellariae are absent. Sexes are separate. Gonads are sim- The other species has a shallow, sac- The upper (oral) surface bears the ply masses of cells in the genital cavity like stomach but no intestine or anus. mouth, which opens into a short esoph- of the arms and pinnules. Gametes agus, from which the long intestine with escape without ducts through a rupture diverticula proceeds aborally for a dis- in the pinnule wall. Brooding occurs Phylogeny and tance and then makes a complete turn in some forms. Doliolaria larvae (Fig- Adaptive Radiation to the anus, which may be on a raised ure 23-10F) are free swimming for a cone (Figure 23-25B). With the aid of time before they become attached and tube feet and mucous nets, crinoids feed metamorphose. Most living crinoids are Phylogeny on small organisms that are caught in from 15 to 30 cm long, but some fossil Despite the existence of an exten- their ambulacral grooves. Ambulacral species had stalks 20 m in length. sive fossil record, there have been grooves are open and ciliated and numerous contesting hypotheses on serve to carry food to the mouth (Fig- echinoderm phylogeny. Based on the ure 23-25B). Tube feet in the form of Class embryological evidence of the bilateral tentacles are also found in the grooves. larvae, there can be little doubt that The water-vascular system has the Concentricycloidea their ancestors were bilateral and that basic echinoderm plan. The nervous Strange little (less than 1 cm diameter), their coelom had three pairs of spaces system has an oral ring and a radial disc-shaped animals (Figure 23-27) (trimeric). Some investigators have nerve that runs to each arm. The abo- were discovered in water over 1000 m held that radial symmetry arose in a ral or entoneural system is more highly deep off New Zealand. Sometimes free-moving echinoderm ancestor and

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CHAPTER 23 Echinoderms 475

Gonad with Ampulla that sessile groups were derived sev- embryos eral times independently from the free- moving ancestors. However, this view does not account for the adaptive sig- nificance of radial symmetry as an Radial Water adaptation for a sessile existence. The nerve canals Tube foot more traditional view is that the first echinoderms were sessile, became Marginal radial as an adaptation to that exis- spines tence, and then gave rise to the free- moving groups. Figure 23-29 is consis- tent with this hypothesis. It views Terminal Marginal evolution of endoskeletal plates with plate plates stereom structure and of external Hydropore ciliary grooves for feeding as early Dorsal view echinoderm (or pre-echinoderm) developments. The extinct carpoids (Figures 23-28A, 23-29) had stereom ossicles but were not radially symmetri- Gonad cal, and the status of their water-vascular system, if any, is uncertain. Some inves- Ring ossicle tigators regard carpoids as a separate

Ventral view subphylum of echinoderms (Homalo- zoa) and consider them closer to chor- Figure 23-27 dates (Calcichordata, p. 494). The fossil Xyloplax spp. (class Concentricycloidea) are bizarre little disc-shaped echinoderms. With their podia helicoplacoids (Figures 23-28B, 23-29) around the margin, they are the only echinoderms not having podia distributed along ambulacral areas. show evidence of three, true ambulacral grooves, and their mouth was on the side of the body. Brachiole Figure 23-28 Attachment to the substratum by A, Dendrocystites, a carpoid the aboral surface would have led to Food groove (subphylum Homalozoa) with radial symmetry and the origin of the one brachiole. Brachioles are Pelmatozoa. Both Cystoidea (extinct) Mouth so called to distinguish them from the heavier arms of and Crinoidea primitively were at- asteroids, ophiuroids and tached to the substratum by an aboral crinoids. This group bore some stalk. An ancestor that became free- characters interpreted as moving and applied its oral surface to chordate in nature. It is called Calcichordata by some the substratum would have given rise investigators (p. 494). to Eleutherozoa. Phylogeny within B, Helicoplacus, a Eleutherozoa is controversial. Most helicoplacoid, had three investigators agree that echinoids and ambulacral areas and holothuroids are related and form a apparently a water-vascular clade, but opinions diverge on the system. It is the sister group to relationship of ophiuroids and aster- Anus modern echinoderms. Stem oids. Figure 23-29 illustrates the view plates that the ophiuroids arose after the clo- A B sure of ambulacral grooves, but this scheme treats evolution of five ambu- lacral rays (arms) in ophiuroids and asteroids as independently evolved. Alternatively, if ophiuroids and aster- oids are a single clade, then closed ambulacral grooves must have evolved separately in ophiuroids and in the common ancestor of echinoids and holothuroids.

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476 PART 3 The Diversity of Animal Life

Classification of Phylum feather stars. Five arms branching at Class Ophiuroidea (o fe-u-roy de-a) Echinodermata base and bearing pinnules; ciliated (Gr. ophis, snake, oura, tail, ambulacral grooves on oral surface eidos, form, ea, characterized by): There are about 6,000 living and 20,000 with tentacle-like tube feet for food brittle stars and basket stars. Star extinct or fossil species of Echinoderma- gathering; spines, madreporite, and shaped, with arms sharply marked off ta. The traditional classification placed all pedicellariae absent. Examples: Ante- from central disc; ambulacral grooves free-moving forms that were oriented don, Comantheria (Figure 23-26). closed, covered by ossicles; tube feet with oral side down in subphylum Subphylum Eleutherozoa (e-lu ther-o- without suckers and not used for loco- Eleutherozoa, containing most living zo a) (Gr. eleutheros, free, not bound, motion; pedicellariae absent; anus species. The other subphylum, Pelmato- z|on, animal). Body form star-shaped, absent. Examples: Ophiura (Figure zoa, contained mostly forms with stems globular, discoidal, or cucumber-shaped; 23-11A), Gorgonocephalus (Figure and oral side up; most extinct classes oral surface directed toward substratum 23-14B). and living Crinoidea belong to this or oral-aboral axis parallel to substratum; Class Echinoidea (ek i-noy de-a) group. Although alternative schemes body with or without arms; ambulacral (Gr. echinos, sea urchin, hedgehog, have strong supporters, cladistic analysis grooves open or closed. eidos, form, ea, characterized by): provides evidence that the two tradition- Class Concentricycloidea (kon- sea urchins, sea biscuits, and sand al subphyla are monophyletic.* The fol- sen tri-sy-kloy de-a) (L. cum, togeth- dollars. More or less globular or disc- lowing includes only groups with living er, centrum, center [having a com- shaped, with no arms; compact skele- members. mon center], Gr. kyklos, circle, ton or test with closely fitting plates; Subphylum Pelmatozoa (pel-ma to- eidos, form, ea, characterized by): movable spines; ambulacral grooves zo a) (Gr. pelmatos; a stalk, z|on, ani- sea daisies. Disc-shaped body, with closed; tube feet with suckers; pedi- mal). Body in form of cup or calyx, marginal spines but no arms; concen- cellariae present. Examples: Arbacia, borne on aboral stalk during part or all trically arranged skeletal plates; ring Strongylocentrotus (Figure 23-15), of life; oral surface directed upward; of suckerless podia near body margin; Lytechinus, Mellita. open ambulacral grooves; madreporite hydropore present; gut present or Class Holothuroidea (hol o-thu- absent; both mouth and anus on oral absent, no anus. Example: Xyloplax roy de-a) (Gr. holothourion, sea surface; several fossil classes plus living (Figure 23-27). cucumber, eidos, form, ea, char- Crinoidea. Class Asteroidea (as ter-oy de-a) acterized by): sea cucumbers. Class Crinoidea (krin-oi de-a) (Gr. (Gr. aster, star, eidos, form, ea, Cucumber-shaped, with no arms; krinon, lily; eidos, form; ea, characterized by): sea stars (star- spines absent; microscopic ossicles characterized by): sea lilies and fish). Star-shaped, with arms not embedded in thick muscular wall; *Brusca, R. C., and G. J. Brusca. 1990. Inverte- sharply marked off from central disc; anus present; ambulacral grooves brates. Sunderland, Massachusetts, Sinauer ambulacral grooves open, with tube closed; tube feet with suckers; circum- Associates; Meglitsch, P. A, and F. R. Schram. feet on oral side; tube feet often with oral tentacles (modified tube feet); 1991. Invertebrate zoology, ed. 3. New York, suckers; anus and madreporite aboral; pedicellariae absent; madreporite inter- Oxford University Press; Paul, C. R. S., and pedicellariae present. Examples: Aste- nal. Examples: Sclerodactyla, Parasti- A. B. Smith. 1984. Biol. Rev. 59:443–481. rias, Pisaster (p. 458). chopus, Cucumaria (Figure 23-21C).

Data on the Concentricycloidea skeleton. If their ancestors had a brain to which they have exploited the are insufficient to place this group on a and specialized sense organs, these hydraulic mechanism of their tube feet. cladogram, although they are tenta- were lost in the adoption of radial The basic body plan of echino- tively placed in Eleutherozoa. symmetry. Thus it is not surprising that derms has severely limited their evolu- there are large numbers of creeping, tionary opportunities to become para- benthic forms with filter-feeding, sites. Indeed, the most mobile of Adaptive Radiation deposit-feeding, scavenging, and her- echinoderms, the ophiuroids, which Radiation of echinoderms has been bivorous habits, comparatively few are also the ones most able to insert determined by limitations and poten- predators, and very rare pelagic forms. their bodies into small spaces, are the tials inherent in their most important In this light the relative success of only group with significant numbers of characters: radial symmetry, water- asteroids as predators is impressive commensal species. vascular system, and dermal endo- and probably attributable to the extent

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CHAPTER 23 Echinoderms 477

Echinodermata Pelmatozoa Eleutherozoa

Homalozoa† Helicoplacoidea† Crinoidea Asteroidea Ophiuroidea Echinoidea Holothuroidea

Loss of podial suckers

Loss of anus 5 arms broadly connected Fusion of skeletal ossicles to central disc to form rigid test Madreporite moved Reduction of ossicles to oral surface Madreporite internal Elongation of oral-aboral axis One or two brachioles Arms with highly with food grooves articulated "vertebrae" Extension of ambulacral grooves from oral to aboral pole Arms with open ambulacral grooves Stem with 2 rows of for suspension feeding Aboral surface reduced to region around anus ossicles (at least in part)

Loss of external madreporite Closed ambulacral grooves

Madreporite on aboral surface Spindle shaped Suckers on podia Oral surface against substratum Ossicles spirally arranged Movement of anus to aboral surface Mouth lateral Attachment to substratum by aboral surface Mouth and anus on oral surface 5 open ambulacral grooves Pentaradial symmetry

3 ambulacral grooves Water vascular system Figure 23-29 Cladogram showing hypothetical relationships among echinoderm groups. The extinct Homalozoa (carpoids), which were not radial in symmetry but had stereom endoskeletal plates, represent an early split from echinoderms. An intermediate form is represented by the extinct helicoplacoids, which had three ambulacral grooves that wound around their bodies in a spiral fashion. Helicoplacoids are the sister group of modern echinoderms. Evolution of pentaradial symmetry was an adaptation to sessile existence and is a synapomorphy of modern echinoderms. The scheme depicted here views ophiuroids as External ciliary grooves for suspension feeding having arisen separately from asteroids, after the evolution of closed ambulacral grooves, Endoskeletal plates with stereom structure and the possession of five arms would thus have been of separate origin. Alternatively, if Asteroidea and Ophiuroidea form a monophyletic group, with five arms being synapomorphic, then closed ambulacral grooves in the ophiuroids would have evolved † Extinct groups. separately from that character in echinoids and holothuroids

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478 PART 3 The Diversity of Animal Life

Summary

Phylum Echinodermata shows the charac- very simple. The bilateral, free-swimming aboral pole. Holothuroids are greatly elon- teristics of the Deuterostomia division of larva becomes attached, transforms to a gated in the oral-aboral axis and lie on their the animal kingdom. They are an important radial juvenile, then detaches and becomes side. Because certain of the ambulacral marine group sharply distinguished from a motile sea star. areas are characteristically against the sub- other phyla of animals. They have radial Arms of brittle stars (class Ophi- stratum, sea cucumbers have also under- symmetry but were derived from bilateral uroidea) are slender and sharply set off gone some return to bilateral symmetry. ancestors. from the central disc. Ophiuroids have no The tube feet around the mouth are modi- Sea stars (class Asteroidea) can be pedicellariae or ampullae and their ambu- fied into tentacles, with which they feed. used to illustrate echinoderms. Sea stars lacral grooves are closed. Their tube feet They have an internal respiratory tree, and usually have five arms, which merge gradu- have no suckers, and their madreporite is the madreporite hangs free in the coelom. ally with a central disc. Like other echino- on the oral side. They crawl by means of Sea lilies and feather stars (class derms, they have no head and few special- arm movements, and their tube feet func- Crinoidea) are the only group of living ized sensory organs. The mouth is directed tion in food gathering. echinoderms, other than asteroids, with toward the substratum. They have stereom Dermal ossicles of sea urchins (class open ambulacral grooves. They are dermal ossicles, respiratory papulae, and Echinoidea) are closely fitting plates, the mucociliary particle feeders and lie with open ambulacral grooves. Many sea stars body is compact, and there are no arms. their oral side up. have pedicellariae. Their water-vascular Ambulacral areas are closed and extend Sea daisies (class Concentricycloidea) system is an elaborate hydraulic system around their body toward the aboral pole. are a newly discovered class of very small derived embryonically from one of their Sea urchins move by means of tube feet or echinoderms that are circular in shape, coelomic compartments. Along the ambu- by their spines. Some urchins (sand dollars have marginal tube feet, and two concentric lacral areas, branches of the water-vascular and heart urchins) have returned to adult ring canals in their water-vascular system. system (tube feet) are important in locomo- bilateral symmetry. Ancestors of echinoderms were bilat- tion, food gathering, respiration, and excre- Dermal ossicles in sea cucumbers erally symmetrical, and they probably tion. Many sea stars are predators, whereas (class Holothuroidea) are very small; there- evolved through a sessile stage that others feed on small particles. Sexes are fore the body wall is soft. Their ambulacral became radially symmetrical and then gave separate, and reproductive systems are areas also are closed and extend toward the rise to free-moving forms.

Review Questions

1. What constellation of characteristics is 8. Compare the structures and functions 11. Define the following: pedicellariae, possessed by echinoderms and is in question 7 as they are found in brit- madreporite, respiratory tree, Aris- found in no other phylum? tle stars, sea urchins, sea cucumbers, totle’s lantern. 2. How do we know that echinoderms and crinoids. 12. What evidence suggests that ancestral were derived from an ancestor with 9. Briefly describe development in sea echinoderms were sessile? bilateral symmetry? stars, including metamorphosis. 13. Give four examples of how echino- 3. Distinguish the following groups of 10. Match the groups in the left column derms are important to humans. echinoderms from each other: with all correct answers in the right 14. What is a major difference in the func- Crinoidea, Asteroidea, Ophiuroidea, column. tion of the coelom in holothurians Echinoidea, Holothuroidea, Concentri- Crinoidea a. Closed ambulacral compared with other echinoderms? cycloidea. grooves 15. Describe a reason for the hypothesis 4. What is an ambulacrum, and what is Asteroidea b. Oral surface that the ancestor of eleutherozoan the difference between open and generally upward groups was a radial, sessile organism. closed ambulacral grooves? Ophiuroidea c. With arms 5. Trace or make a rough copy of Figure d. Without arms 23-3B without labels; then from mem- Echinoidea e. Approximately glob- ory label the parts of the water- ular or disc-shaped vascular system of sea stars. Holothuroidea f. Elongated in oral- 6. Briefly explain the mechanism of aboral axis Concentri- action of a sea star’s tube foot. g. With pedicellariae cycloidea 7. What structures are involved in the fol- h. Madreporite internal lowing functions in sea stars? Briefly i. Madreporite on oral describe the action of each: respira- plate tion, feeding and digestion, excretion, reproduction.

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CHAPTER 23 Echinoderms 479

Selected References

Baker, A. N., F. W. E. Rowe, and H. E. S. Clark. allies: Echinoderms of Florida and the February 1996. Coral Reefs. 15:209–210. 1986. A new class of Echinodermata from Caribbean. Washington, Smithsonian Insti- This is the first report of an outbreak of New Zealand. Nature 321:862–864. tution Press. An excellent field guide for Acanthaster planci in Indonesia. It includes Describes the strange class Concentricy- echinoderm identification. a good photograph of an aggregation of cloidea. Hickman, C. P., Jr. 1998. A field guide to sea these sea stars. Birkeland, C. 1989. The Faustian traits of the stars and other echinoderms of Galápagos. Lawrence, J. 1989. A functional biology of crown-of-thorns starfish. Am. Sci. Lexington, VA, Sugar Spring Press. Provides echinoderms. Baltimore, The Johns Hop- 77:154–163. Fast growth in the early years descriptions and nice photographs of mem- kins Press. Well-researched book on echin- of the life of an Acanthaster planci results bers of classes Asteroidea, Ophiuroidea, oderm biology with emphasis on feeding, in loss of body integrity in later life. Echinoidea, and Holothuroidea in the maintenance, and reproduction. Gilbert, S. F. 1997. Developmental biology, ed. Galápagos Islands. Moran, P. J. 1990. Acanthaster planci (L.): bio- 5. Sunderland, Massachusetts, Sinauer Hughes, T. P. 1994. Catastrophes, phase shifts graphical data. Coral Reefs 9:95–96. Pre- Associates. Any modern text in develop- and large-scale degradation of a Caribbean sents a summary of essential biological mental biology, such as this one, provides a coral reef. Science 265:1547–1551. data on A. planci. This entire issue of Coral multitude of examples in which studies on Describes the sequence of events, including Reefs is devoted to A. planci. echinoderms have contributed (and con- the die-off of sea urchins, leading to the tinue to contribute) to our knowledge of destruction of the coral reefs around development. Jamaica. Hendler, G., J. E. Miller, D. L. Pawson, and P. M. Lane, D. J. W. 1996. A crown-of-thorns outbreak Kier. 1995. Sea stars, sea urchins, and in the eastern Indonesian Archipelago,

Zoology Links to the Internet

Visit the textbook’s web site at The CAS Echinoderm Webpage. Informa- Phylum Echinodermata, from the Universi- www.mhhe.com/zoology to find live tion on echinoderm taxonomy and on the ty of Minnesota. Internet links for each of the references echinoderm collection of the California · Dissection of a Starfish. below. Academy of Science. It also provides links Animal Diversity Web, University of Michi- to other echinoderm sites. Starfish, External and Internal Anatomy. gan. Phylum Echinodermata. Information The Echinoderm Newsletter. This news- Echinoderms. Keys to Marine Invertebrates about echinoderms, with links to various letter, prepared by the National Museum of the Woods Hole Region. Descriptive groups of echinoderms. Nice pictures: of Natural History, provides information information, definition of terminology, and check out the magnificent urchin, Astropy- on conferences and publications on echin- keys to the echinoderms of the Woods ga magnifica. It’s obvious why it was so oderms, and gives addresses of biologists Hole Region. named! studying echinoderms. Echinoderms, University of Minnesota. Introduction to the Echinoderms. Univer- Echinodermata. Arizona’s Tree of Life Web Information about echinoderms, and a link sity of California at Berkeley Museum of Page. Pictures, characteristics, phylogenetic to the sea star dissection home page. Paleontology site provides information on relationships, references on echinoderms. the echinoderm fossil record, life histories, systematics, and morphology. It also pro- vides a great number of links to sites that focus on each of the echinoderm classes. OLC Student Center Website

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C H A P T E R

24

Chaetognaths and Hemichordates Phylum Chaetognatha Phylum Hemichordata

An enteropneust hemichordate.

Part Chordates Unfortunately, the structure that Bateson interpreted as In the mid-nineteenth century, with interest in the origin of a notochord neither looks like a notochord nor develops

the chordates running high, a group of wormlike marine like a notochord. These and other problems with giving the invertebrates of unknown relationship began to attract atten- hemichordates membership in the chordate club were noted tion with their chordatelike characteristics. In 1885 W. Bate- in the 1930s, but by this time the concept had become son named them Hemichordata and forcefully argued that firmly established in textbooks and began to assume a life of these organisms should be included in phylum Chordata. its own. Some zoologists and texts doggedly continued to Bateson pointed to several hemichordate structures that he assign subphylum Hemichordata to phylum Chordata for 25 believed were homologous with comparable features in or more years. Eventually most zoologists agreed that hemi- chordates: a dorsal nerve cord, gill slits, and, most important- chordates should be viewed as a distinct phylum of ―lesser‖ ly, a sac-like evagination of the mouth region that he inter- deuterostomes. Bateson’s name—Hemichordata—has stuck, preted as a notochord. The notochord, a rodlike, supportive however, and oddly enough seems appropriate for a group

structure lying dorsal to the gut of early growth stages of all of animals that, although lacking a notochord, does bear chordates, is a key distinguishing feature of the phylum certain characters in common with true chordates. As the Chordata. If the hemichordates possessed a notochord— likely sister group of chordates, hemichordates are indeed even half a notochord—they had to be chordates. half (or part) chordates. I

480

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CHAPTER 24 Chaetognaths and Hemichordates 481

Deuterostomes include, along with Grasping Echinodermata two other phyla: Hemi- spines chordata and Chordata. Two chordate subphyla—Urochordata and Cephalo- Mouth chordata—are also invertebrate groups. Phylum Chaetognatha traditionally has Central been included among deuterostomes, ganglion but this arrangement is not supported A by recent molecular evidence.* Chae- tognaths do bear a number of deutero- stome characters, however, and we will Intestine continue to include them in this chapter for the present. These phyla have en- terocoelous development of the coelom Ovary and some form of radial cleavage. Fins

Phylum Oviduct Chaetognatha Anus Testis Female A common name for chaetognaths is gonopore B arrowworms. They are all marine ani- Seminal mals and are highly specialized for vesicle their planktonic existence. Their rela- Caudal fin tionship to other groups is obscure, although embryological characters indi- Figure 24-1 cate deuterostome affinities. Arrowworms. A, Internal structure of Sagitta. B, Scanning electron micrograph of a juvenile The name Chaetognatha (ke- arrowworm, Flaccisagitta hexaptera (35 mm length) eating a larval fish. tog na-tha) (Gr. chaitƒ, long flowing hair, gnathos, jaw) refers to the sickle-shaped bristles on each side of A thin cuticle covers the body, and the mouth. This is not a large group, Form and Function the epidermis is single layered except for only some 65 species are known. The body of an arrowworm is unseg- along the sides of the body, where it is Their small, straight bodies resemble mented and includes a head, trunk, stratified in a thick layer. These are the miniature torpedoes, or darts, ranging and postanal tail (Figure 24-1A). On only invertebrates with a many-layered from 2.5 to 10 cm in length. the underside of the head is a large epidermis. Arrowworms are all adapted for a vestibule leading to the mouth. The Arrowworms have a complete planktonic existence, except for vestibule contains teeth and is flanked digestive system, a well-developed Spadella (Gr. spadix, palm frond, on both sides by curved chitinous coelom, and a nervous system with a ella, dim. suffix), a benthic genus. They spines used in seizing prey. A pair of nerve ring containing large dorsal and usually swim to the surface at night and eyes is on the dorsal side. A peculiar ventral ganglia and a number of lateral descend during the day. Much of the hood formed from a fold of the neck ganglia. Sense organs include eyes, time they drift passively, but they can can be drawn forward over the head sensory bristles, and a unique U- dart forward in swift spurts, using the and spines. When the animal captures shaped ciliary loop that extends over caudal fin and longitudinal muscles— prey, it retracts the hood, and the teeth the neck from the back of the head. a fact that no doubt contributes to and raptorial spines spread apart and The ciliary loop may detect water cur- their success as planktonic predators. then snap shut with startling speed. rents or may be chemosensory. How- Horizontal fins bordering the trunk are Arrowworms are voracious feeders, ever, vascular, respiratory, and excre- used in flotation rather than in active living on planktonic forms, especially tory systems are entirely lacking. swimming. copepods, and even small fish (Fig- Arrowworms are hermaphroditic

ure 24-1B). When they are abundant, with either cross- or self-fertilization. as they often are, they may have a sub- Eggs of Sagitta (L. arrow) bear a coat stantial ecological impact. They are of jelly and are planktonic. Eggs of nearly transparent, a characteristic of other arrowworms may be attached to *Telford, M. J., and P. W. H. Holland. 1993. Mol. Biol. Evol. 10:660–676; Wada, H., and N. Satoh. 1994. adaptive value in their role as plank- the body and carried about for a time. Proc. Natl. Acad. Sci. 91:1801–1804. tonic predators. Juveniles develop directly without

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482 PART 3 The Diversity of Animal Life

Proboscis

Position in Animal Proboscis stalk Kingdom

1. Hemichordates belong to the Gill pores deuterostome branch of the animal A Trunk kingdom and are enterocoelous coelomates with radial cleavage. 2. A chordate plan of structure is Mouth suggested by gill slits and a Collar restricted dorsal tubular nerve cord. Proboscis pore Proboscis 3. Similarity to echinoderms is shown Collar coelom in larval characteristics. Circular muscle Glomerulus Collar Collar nerve cord Heart Trunk nerve cord Biological Contributions Dorsal blood vessel 1. A tubular dorsal nerve cord in the collar zone may represent an early stage of the condition in chordates; a diffused net of nerve B cells is similar to the uncentralized, subepithelial plexus of echino- derms. 2. Gill slits in the pharynx, which are also characteristic of chordates, serve primarily for filter feeding Proboscis coelom Mouth Gill slits and only secondarily for breathing Longitudinal Skeletal plate Pharynx and are thus comparable to those muscle Buccal Buccal Ventral Ventral in protochordates. diverticulum cavity vessel nerve cord Figure 24-2 Acorn worm Saccoglossus (Hemichordata, class Enteropneusta). A, External lateral view. metamorphosis. Chaetognath embryo- B, Longitudinal section through anterior end. genesis differs from that of typical deuterostomes in that the coelom is formed by a backward extension from with the chordate notochord, so hemi- Class Enteropneusta the archenteron rather than by chordates are given the rank of a sepa- Enteropneusts, or acorn worms, are pinched-off coelomic sacs. There is no rate phylum. sluggish, wormlike animals that live in true peritoneum lining the coelom. Hemichordates are vermiform bot- burrows or under stones, usually in Cleavage is radial, complete, and tom dwellers, living usually in shallow mud or sand flats of intertidal zones. equal. waters. Some colonial species live in Balanoglossus (Gr. balanos, acorn, A common arrowworm is Sagitta secreted tubes. Most are sedentary or gl|ssa, tongue) and Saccoglossus (Gr. (Figure 24-1A). sessile. Their distribution is almost cos- sakkos, sac, strainer, gl|ssa, tongue) mopolitan, but their secretive habits (Figure 24-2) are common genera. and fragile bodies make collecting Phylum them difficult. Members of class Enteropneusta Form and Function Hemichordata (Gr. enteron, intestine, pneustikos, The mucus-covered body is divided Hemichordata (hem i-kor-da ta) (Gr. of, or for, breathing) (acorn worms) into a tonguelike proboscis, a short hemi, half, chorda, string, cord) are range from 20 mm to 2.5 m in length. collar, and a long trunk (protosome, marine animals that were formerly con- Members of class Pterobranchia (Gr. mesosome, and metasome). sidered a subphylum of chordates, pteron, wing, branchia, gills) are based on their possession of gill slits smaller, usually 1 to 12 mm, not Proboscis The proboscis is the active and a rudimentary notochord. How- including the stalk. About 70 species of part of the animal. It probes about in ever, the so-called hemichordate noto- enteropneusts and two small genera of the mud, examining its surroundings chord is really a buccal diverticulum pterobranchs are recognized. and collecting food in mucous strands (called a stomochord, meaning Hemichordates have the typical tri- on its surface. Cilia carry particles to the ―mouth-cord‖) and not homologous coelomate structure of deuterostomes. groove at the edge of the collar, direct

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CHAPTER 24 Chaetognaths and Hemichordates 483

A on the gill slits, but some respiratory Characteristics of gaseous exchange occurs in the vascu-

Phylum Hemichordata lar branchial epithelium, as well as in the body surface. Ciliary currents keep 1. Soft bodied; wormlike or short and a fresh supply of water moving from compact with stalk for attachment the mouth through the pharynx and Mouth open Gill pores 2. Body divided into proboscis, col- out the gill slits and branchial cham- in feeding lar, and trunk; coelomic pouch single in proboscis, but paired in bers to the outside. other two; buccal diverticulum in posterior part of proboscis B Feeding and the Digestive System 3. Enteropneusta free moving and of Hemichordates are largely ciliary-mucus burrowing habits; pterobranchs feeders. Behind the buccal cavity lies sessile, mostly colonial, living in the large pharynx containing in its dor- secreted tubes sal part the U-shaped gill slits (Fig- 4. Circulatory system of dorsal and ure 24-2B). Since there are no gills, the ventral vessels and dorsal heart primary function of the branchial mech- Mouth occluded 5. Respiratory system of gill slits anism of the pharynx is presumably Figure 24-3 (few or none in pterobranchs) food gathering. Having been caught in Food currents of enteropneust hemichordate. connecting the pharynx with out- A, Side view of acorn worm with mouth open, side as in chordates mucus and brought to the mouth by cil- showing direction of currents created by cilia on 6. No nephridia; a single glomeru- iary action on the proboscis and collar, proboscis and collar. Food particles are directed lus connected to blood vessels food particles are strained from the toward mouth and digestive tract. Rejected may have excretory function branchial water that leaves through the particles move toward outside of collar. Water 7. A subepidermal nerve plexus gill slits. Food then passes to the ventral leaves through gill pores. B, When mouth is thickened to form dorsal and ven- part of the pharynx and esophagus to occluded, all particles are rejected and passed onto the collar. Nonburrowing and some tral nerve cords, with a ring con- the intestine, where digestion and burrowing hemichordates use this feeding nective in the collar; dorsal nerve absorption occur (Figure 24-3). method. cord of collar hollow in some 8. Sexes separate in Enteropneusta, with gonads projecting into body Circulatory and Excretory Systems them to the mouth on the underside, cavity; in pterobranchs reproduc- A middorsal vessel carries the colorless and then the particles are swallowed. tion may be sexual or asexual (in blood forward above the gut. In the Large particles can be rejected by cov- some) by budding; tornaria larva collar the vessel expands into a sinus ering the mouth with the edge of the in some Enteropneusta and a heart vesicle above the buccal diverticulum. Blood then enters a net- collar (Figure 24-3). Burrow dwellers use the proboscis work of blood sinuses called the to excavate, thrusting it into the mud glomerulus, which partially surrounds or sand and allowing cilia and mucus slender canal connects the protocoel these structures. The glomerulus is to move the sand backward. Or they with a proboscis pore to the outside assumed to have an excretory function may ingest sand or mud as they go, (Figure 24-2B). The paired coelomic (Figure 24-2B). Blood travels poste- extracting its organic contents. They cavities in the collar also open by riorly through a ventral vessel below build U-shaped, mucus-lined burrows, pores. By taking in water through the the gut, passing through extensive usually with two openings 10 to 30 cm pores into the coelomic sacs, the pro- sinuses to the gut and body wall. apart and with the base of the U 50 to boscis and collar can be stiffened to 75 cm below the surface. They can aid in burrowing. Contraction of the Nervous and Sensory Systems The thrust their proboscis out the front body musculature then forces the nervous system consists mostly of a sub- opening for feeding. Defecation at the excess water out through the gill slits, epithelial network, or plexus, of nerve back opening builds characteristic spi- reducing the hydrostatic pressure and cells and fibers to which processes of ral mounds of feces that leave a telltale allowing the animal to move forward. epithelial cells are attached. Thicken- clue to the location of burrows. ings of this net form dorsal and ventral In the posterior end of the pro- Branchial System A row of gill nerve cords that are united posterior to boscis is a small coelomic sac (proto- pores is located dorsolaterally on each the collar by a ring connective. The coel) into which extends the buccal side of the trunk just behind the collar dorsal cord continues into the collar diverticulum, a slender, blindly end- (Figure 24-3A). Pores open from a and furnishes many fibers to the plexus ing pouch of the gut that reaches for- series of gill chambers that in turn con- of the proboscis. The collar cord is hol- ward into the buccal region and was nect with a series of gill slits in the low in some species and contains giant formerly considered a notochord. A sides of the pharynx. There are no gills nerve cells with processes running to

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484 PART 3 The Diversity of Animal Life

Ciliated tract Coelom1 Ciliated (protocoel) tract

Pharyngeal Mouth slits Coelom1&2 Mouth Coelom 2 (axohydrocoel)

Coelom3 Stomach Anus Ciliated Coelom3 ring (somatocoel) A B Anus

Tornaria Early bipinnaria Figure 24-4 Comparison of a hemichordate tornaria (A) to an echinoderm bipinnaria (B).

the nerve trunks. This nerve plexus sys- (Cephalodiscus and Rhabdopleura) are tem is quite reminiscent of that of known in any detail. cnidarians and echinoderms. Pterobranchs are small animals, Tentacle Sensory receptors include neu- usually within the range of 1 to 7 mm in Arm rosensory cells throughout the epider- length, although the stalk may be mis (especially in the proboscis, a longer. Many individuals of Cephalodis- preoral ciliary organ that may be cus (Gr. kephalƒ, head, diskos, disc) Collar chemoreceptive) and photoreceptor (Figure 24-5) live together in collage- cells. Gill slit nous tubes, which often form an anasto- mosing system. Zooids are not con- Cephalic Reproductive System and Develop- nected, however, and live Trunk shield ment Sexes are separate in enterop- independently in the tubes. Through neusts. A dorsolateral row of gonads apertures in these tubes, they extend Tube runs along each side of the anterior their crown of tentacles. They are part of the trunk. Fertilization is exter- attached to the walls of the tubes by Stalk nal, and in some species a ciliated extensible stalks that can jerk the own- tornaria larva develops that at certain ers back into the tubes when necessary. stages is so similar to the echinoderm The body of Cephalodiscus is bipinnaria that it was once believed to divided into the three regions—pro- be an echinoderm larva (Figure 24-4). boscis, collar, and trunk—characteristic The familiar Saccoglossus of American of hemichordates. There is only one waters has direct development without pair of gill slits, and the alimentary U-shaped, a tornaria stage. canal is with the anus near the mouth. The proboscis is shield shaped. At the base of the proboscis Class Pterobranchia are five to nine pairs of branching arms The basic plan of class Pterobranchia is with tentacles containing an extension similar to that of Enteropneusta, but of the coelomic compartment of the certain structural differences are corre- mesosome, as in a lophophore. Cili- lated with the sedentary life-style of ated grooves on the tentacles and arms pterobranchs. The first pterobranch collect food. Some species are dioe- Cephalodiscus colony ever reported was obtained by the cious, and others are monoecious. Figure 24-5 famed Challenger expedition of 1872 Asexual reproduction by budding may to 1876. Although first placed among Cephalodiscus, a pterobranch hemichordate. also occur. These tiny (5 to 7 mm) forms live in tubes in Polyzoa (Entoprocta and Ectoprocta), In Rhabdopleura (Gr. rhabdos, which they can move freely. Ciliated tentacles its affinities to hemichordates were rod, pleura, a rib, the side), which and arms direct currents of food and water later recognized. Only two genera is smaller than Cephalodiscus, the toward mouth.

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CHAPTER 24 Chaetognaths and Hemichordates 485

A B Figure 24-6 A, Rhabdopleura, a pterobranch hemichordate in its tube. Individuals live in branching tubes connected by stolons, and protrude the ciliated tentacles for feeding. B, Portion of a colony.

members remain together to form a characters with both echinoderms and clade (Echinodermata, Hemichordata, colony of zooids connected by a stolon chordates. With chordates they share and Chordata) and places the and enclosed in secreted tubes (Fig- gill slits, which serve primarily for filter lophophorate phyla in superphylum ure 24-6). The collar in these forms feeding and secondarily for breathing, Lophotrochozoa of Protostomia. bears two branching arms. No gill as they do in some protochordates. In Other than their shared deutero- clefts or glomeruli are present. New addition, a short dorsal, somewhat hol- stome characters, the relationship of individuals are produced by budding low nerve cord in the collar zone may chaetognaths to deuterostome phyla is from a creeping basal stolon, which be homologous to the nerve cord of enigmatic. Sequence analysis of the branches on a substratum. No ptero- chordates (Figure 24-7). The buccal gene encoding small-subunit rRNA branch has a tubular nerve cord in the diverticulum in the hemichordate supports placement of chaetognaths collar, but otherwise their nervous sys- mouth cavity, long thought homolo- among protostomes. Some investiga- tem is similar to that of Enteropneusta. gous to the notochord of chordates, is tors suggest, however, that chae- The fossil graptolites of the middle now considered a synapomorphy tognaths are neither protostomes nor Paleozoic era often are placed as an of hemichordates themselves. Early deuterostomes but originated indepen- extinct class under Hemichordata. embryogenesis of hemichordates is dently from an early coelomate lineage. They are important index fossils of the remarkably like that of echinoderms, Ordovician and Silurian geological and the early tornaria larva is almost strata. Alignment of graptolites with identical to the bipinnaria larva of Adaptive Radiation the hemichordates has been very con- asteroids, suggesting that echinoderms Because of their sessile lives and their troversial, but discovery of an organ- form the sister group of hemichordates habitat in secreted tubes in ocean bot- ism that seems to be a living graptolite and chordates (Figure 24-7). However, toms, where conditions are fairly sta- lends strong support to the hypothesis. Brusca and Brusca* placed lophophor- ble, pterobranchs have undergone lit- It has been described as a new species ates as the sister group of hemichor- tle adaptive divergence. They have of pterobranch, called Cephalodiscus dates and chordates, required by their retained a tentacular type of ciliary graptolitoides. proposed synapomorphy for all these feeding. Enteropneusts, on the other groups of a crown of ciliated tentacles hand, although sluggish, are more containing extensions of the mesocoel. active than pterobranchs. Having lost Phylogeny and Their hypothesis is not supported by their tentaculated arms, they use a pro- Adaptive Radiation analysis of the base sequence of the boscis to trap small organisms in gene encoding the small-subunit of mucus, or they eat sand as they bur- rRNA, which indicates a deuterostome row and digest organic sediments from Phylogeny the sand. Their evolutionary diver- Hemichordate phylogeny has long *Brusca, R. C., and G. J. Brusca. 1990. Invertebrates. gence, although greater than that of been puzzling. Hemichordates share Sunderland, Massachusetts, Sinauer Associates. pterobranchs, is still modest.

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486 PART 3 The Diversity of Animal Life

Deuterostomia Hemichordata

Echinodermata Enteropneusta Pterobranchia Chordata

Burrowing life-styles Sessile, colonial External ciliary grooves for life-styles suspension feeding Notochord Water-vascular system from mesocoel Postanal tail Unique excretory structure: glomerulus Endostyle or thymus Hemal system Preoral gut Loss of mesosomal tentacles diverticulum Tadpole larva Endoskeleton of stereom ossicles

Radial symmetry Dorsal hollow nerve cord Pharyngeal gill slits

Crown of ciliated tentacles containing mesocoel

Enterocoely Mesoderm derived from archenteron Mouth not derived from blastopore Coelom tripartite (or derived therefrom)

Figure 24-7 Cladogram showing hypothetical relationships among deuterostome phyla. Brusca and Brusca considered the crown of ciliated tentacles (containing extensions of the mesocoel) a character borne by ancestors of lophophorates, hemichordates and chordates. The tentacular crown would have become the lophophore in lophophorate phyla and retained as a primitive character in pterobranchs. Because molecular evidence indicates that lophophorates are protostomes, we removed them from this cladogram; the ciliated tentacular crown in pterobranchs and lophophorates can be considered a convergent character. Source: Modified from R. C. Brusca and G. S. Brusca, Invertebrate. Sinauer Associates, Inc., Sunderland, MA., 1990.

Summary

Arrowworms (phylum Chaetognatha) are a ered chordates because their buccal diver- that feed on particles strained out of the small group but an important component ticulum was considered a notochord. How- water by gill slits. Members of class Ptero- of marine plankton. They have a well- ever, like chordates, some of them do have branchia are tube dwellers, filter feeding developed coelom and are effective preda- gill slits and a hollow, dorsal nerve cord. with tentacles. Hemichordates are impor- tors, catching other planktonic organisms Divisions of their body (proboscis, collar, tant phylogenetically because they show with the teeth and chitinous spines around trunk) contain the typical deuterostome affinities with chordates and echinoderms, their mouth. coelomic compartments (protocoel, meso- and they are the likely sister group of chor- Members of phylum Hemichordata are coel, metacoel). The hemichordate class marine worms that were formerly consid- Enteropneusta contains burrowing worms dates.

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CHAPTER 24 Chaetognaths and Hemichordates 487

Review Questions

1. What is evidence that Chaetognatha are 3. What characteristics do Hemichordata 5. What is the evidence that Hemichordata deuterostomes? What is evidence that share with Chordata, and how do the are related to echinoderms? conflicts with this hypothesis? two phyla differ? 2. What is the ecological importance of 4. Distinguish Enteropneusta from Ptero- arrowworms? branchia.

Selected References

Barrington, E. J. W. 1965. The biology of Hemi- a dorsal nervous system, a characteristic discovery of the “living fossil,” Cephalodis- chordata and Protochordata. San Francisco, shared in the animal kingdom only with cus graptolitoides. W. H. Freeman & Company. Concise Hemichordata and Chordata. The authors Thuesen, E. V., and K. Kogure, 1989. Bacterial account of behavior, physiology, and repro- contend that the character in chaetognaths production of tetrodotoxin in four species duction of hemichordates, urochordates, is convergent with that in hemichordates of Chaetognatha. Biol. Bull. 176:191–194. and cephalochordates. and chordates. Chaetognaths use venom to enhance prey Bieri, R., and E. V. Thuesen. 1990. The strange Svitii, K. A. 1993. It’s alive, and it’s a graptolite. capture, and the venom (tetrodotoxin) is worm Bathybelos. Am. Sci. 78:542–549. Discover 14(7):18–19. Short account of the produced by bacteria (Vibrio alginolyticus). Bathybelos is a peculiar chaetognath with

Zoology Links to the Internet

Visit the textbook’s web site at istics of chordates, with the following link Introduction to the Hemichordata. Univer- www.mhhe.com/zoology to find live to urochordates and vertebrates. sity of California at Berkeley Museum of Internet links for each of the references Chordata. Arizona’s Tree of Life Web Page. Paleontology site provides photographs below. and information on the biology and classi- An introduction, pictures, characteristics, fication of the hemichordates. Animal Diversity Web, University of Michi- phylogenetic relationships, and references gan. Phylum Chordata. General character- on chordates. Graptolites. Photos and information on these relatives of hemichordates.

OLC Student Center Website

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C H A P T E R

251

Chordates General Characteristics, Protochordates, and Ancestry of the Earliest Vertebrates

Two amphioxus in feeding posture.

It’s a Long Way From Amphioxus It’s a long way from amphioxus Along the more southern coasts of North America, half It’s a long way to us. buried in sand on the sea floor, lives a small fishlike trans- It’s a long way from amphioxus lucent animal quietly filtering organic particles from sea- To the meanest human cuss. water. Inconspicuous, of no commercial value and largely Well, it’s good-bye to fins and gill slits unknown, this creature is nonetheless one of the famous ani- And its welcome lungs and hair, mals of classical zoology. It is amphioxus, an animal that It’s a long, long way from amphioxus wonderfully exhibits the four distinctive hallmarks of the But we all came from there. phylum Chordata—(1) dorsal, tubular nerve cord overlying (2) a supportive notochord, (3) pharyngeal slits for filter But amphioxus’ place in the sun was not to endure. For one feeding, and (4) a postanal tail for propulsion—all wrapped thing, amphioxus lacks one of the most important of verte- up in one creature with textbook simplicity. Amphioxus is an brate characteristics, a distinct head with special sense animal that might have been designed by a zoologist for the organs and the equipment for shifting to an active predatory classroom. During the nineteenth century, with interest in mode of life. Absence of a head, together with several spe- vertebrate ancestry running high, amphioxus was considered cialized features, suggests to zoologists today that amphi- by many to resemble closely the direct ancestor of the verte- oxus represents an early departure from the main line of brates. Its exalted position was later acknowledged by Philip chordate descent. It seems that we are a very long way Pope in a poem sung to the tune of ―Tipperary.‖ It ends with indeed from amphioxus. Nevertheless, while amphioxus is the refrain: denied the vertebrate ancestral award, we believe that it more closely resembles the earliest prevertebrate than any other living animal we know. I

488 488

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CHAPTER 25 Chordates 489

Notochord Position in the Animal Characteristics of Kingdom Phylum Chordata Phylum Chordata (kor-da ta) (L. chor- 1. Bilateral symmetry; segmented da, cord) belongs to the Deuterosto- Fibrous sheath Elastic sheath body; three germ layers; well- mia branch of the animal kingdom developed coelom that includes the phyla Echinoder- A 2. Notochord (a skeletal rod) pres- mata, Hemichordata, and the three ent at some stage in the life cycle lophophorate phyla—Phoronida, 3. Single, dorsal, tubular nerve Ectoprocta, and Brachiopoda. These cord; anterior end of cord usually six phyla share many embryological enlarged to form brain features and are probably descended Notochord 4. Pharyngeal pouches present at from an ancient common ancestor. some stage in the life cycle; in From humble beginnings, the chor- B aquatic chordates these develop dates have evolved a vertebrate body into pharyngeal slits plan of enormous adaptability that 5. Postanal tail, usually projecting always remains distinctive, while it beyond the anus at some stage but provides almost unlimited scope for may or may not persist specialization in life habitat, form, Figure 25-1 6. Segmented muscles in an unseg- and function. A, Structure of the notochord and its surrounding sheaths. Cells of the notochord mented trunk proper are thick walled, pressed together 7. Ventral heart, with dorsal and Biological Contributions closely, and filled with semifluid. Stiffness is ventral blood vessels; closed blood 1. The endoskeleton of vertebrates caused mainly by turgidity of fluid-filled cells and system permits continuous growth with- surrounding connective tissue sheaths. This 8. Complete digestive system primitive type of endoskeleton is characteristic out molting and attainment of 9. A cartilaginous or bony of all chordates at some stage of the life cycle. large body size, and it provides an The notochord provides longitudinal stiffening of endoskeleton present in the efficient framework for muscle the main body axis, a base for trunk muscles, majority of members (vertebrates) attachment. and an axis around which the vertebral column 2. The perforated pharynx of pro- develops. B, In hagfishes and lampreys it tochordates that originated as a persists throughout life, but in other vertebrates suspension-feeding device served it is largely replaced by vertebrae. In mammals ever, the exact phylogenetic position as the framework for subsequent slight remnants are found in nuclei pulposi of of the chordates within the animal evolution of true internal gills intervertebral discs. The method of notochord kingdom is unclear. with pharyngeal muscular pump, formation is different in the various groups of Two possible lines of descent have animals. In amphioxus it originates from and jaws. endoderm; in birds and mammals it arises as an been proposed. Earlier speculations that 3. Adoption of a predatory habit by anterior outgrowth of the embryonic primitive focused on the arthropod-annelid- the early vertebrates and accompa- streak. mollusc group (Protostomia branch) of nying evolution of a highly dif- the invertebrates have fallen from favor. ferentiated brain and paired It is now believed that only members of special sense organs contributed chord (Gr. n|ton, back, L. chorda, the echinodermhemichordate assem- in large measure to the successful adaptive radiation of vertebrates. cord) (Figure 25-1). All members of the blage (Deuterostomia branch) deserve 4. Paired appendages that appeared phylum possess this structure, either serious consideration as a chordate sis- in the aquatic vertebrates were restricted to early development or pres- ter group. Chordates share with the successfully adapted later as joint- ent throughout life. The notochord is a other Deuterostomes several important ed limbs for efficient locomotion rodlike, semirigid body of cells enclosed characteristics: radial cleavage (p. 162), on land or as wings for flight. by a fibrous sheath, which extends, in anus derived from the first embryonic most cases, the length of the body opening (blastopore) and mouth between the gut tract and central ner- derived from an opening of secondary vous system. Its primary purpose is to origin, and a coelom primitively formed support and stiffen the body, that is, to by fusion of enterocoelous pouches The Chordates act as a skeletal axis. (except in vertebrates in which the The animals most familiar to most peo- The structural plan of chordates coelom is basically schizocoelous). ple belong to the phylum Chordata shares features of many nonchordate These common characteristics indicate a (kor-da ta) (L. chorda, cord). Humans invertebrates, such as bilateral sym- natural unity among the Deuterostomia. are members and share with other chor- metry, anteroposterior axis, coelom, As a whole, there is more funda- dates the characteristic from which the tube-within-a-tube arrangement, me- mental unity of plan throughout all the phylum derives its name—the noto- tamerism, and cephalization. How- organs and systems of this phylum

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490 PART 3 The Diversity of Animal Life than there is in many other phyla. Eco- different kinds of information. The ians). The Gnathostomata in turn can logically the chordates are among the cladogram shows a nested hierarchy of be subdivided into Pisces, jawed verte- most adaptable of organic forms and taxa grouped by their sharing of brates with limbs (if any) in the shape are able to occupy most kinds of habi- derived characters. These characters of fins; and Tetrapoda (Gr. tetras, four, tat. They illustrate perhaps better than may be morphological, physiological, podos, foot), jawed vertebrates with any other animal group the basic evo- embryological, behavioral, chromo- two pairs of limbs. Note that several of lutionary processes of the origin of somal, or molecular in nature. these groupings are paraphyletic (Pro- new structures, adaptive strategies, and Although the cladogram shows the rel- tochordata, Acraniata, Agnatha, Anam- adaptive radiation. ative time of origin of the novel proper- niota, Pisces) and consequently are not ties of taxonomic groups and their spe- accepted in cladistic classifications. Traditional and Cladistic cific positions in the hierarchical system Accepted monophyletic taxa are Classification of the of evolutionary common descent, it shown at the top of the cladogram in contains no timescale or information Figure 25-3 as a nested hierarchy of Chordates on ancestral lineages. By contrast, the increasingly more inclusive groupings. The traditional Linnaean classification of branches of a phylogenetic tree are the chordates (p. 503) provides a simple intended to represent real lineages that and convenient way to indicate the taxa occurred in the evolutionary past. Geo- Four Chordate included in each major group. However, logical information regarding ages of Hallmarks in cladistic usage, some of the traditional lineages is added to information from taxa, such as Agnatha and Reptilia, are the cladogram to generate a phyloge- The four distinctive characteristics that, no longer recognized. Such taxa do not netic tree for the same taxa. taken together, set chordates apart satisfy the requirement of cladistics that In our treatment of the chordates, from all other phyla are notochord, only monophyletic groups are valid we have retained the traditional Lin- dorsal tubular nerve cord, pharyn- taxonomic entities, that is, groups that naean classification (p. 503) because geal pouches, and postanal tail. contain all known descendants of a sin- of its conceptual usefulness and be- These characteristics are always found gle common ancestor. The reptiles, for cause the alternative—thorough revi- at some embryonic stage, although example, are considered a paraphyletic sion following cladistic principles— they may be altered or may disappear grouping because this group does not would require extensive change and in later stages of the life cycle. contain all of the descendants of their virtual abandonment of familiar rank- most recent common ancestor (p. 563). ings. However, we have tried to use Notochord The common ancestor of reptiles as tra- monophyletic taxa as much as possi- ditionally recognized is also the ancestor ble, because such usage is consistent of birds and mammals. As shown in the with both evolutionary and cladistic cladogram (Figure 25-3), reptiles, birds, taxonomy (see p. 201). and mammals compose a monophyletic Several traditional divisions of the clade called Amniota, so named because phylum Chordata used in Linnaean Notochord all develop from an egg having special classifications are shown in Table 25-1. The notochord is a flexible, rodlike extraembryonic membranes, one of A fundamental separation is Proto- structure, extending the length of the which is the amnion. Therefore accord- chordata from Vertebrata. Since the for- body. It is the first part of the endo- ing to cladistics, the reptiles can be mer lack a well-developed head, they skeleton to appear in the embryo. The grouped only in a negative manner as are also called Acraniata. All verte- notochord is an axis for muscle attach- amniotes that are not birds or mammals; brates have a well-developed skull case ment, and because it can bend without there are no positive or novel features enclosing the brain and are called Cra- shortening, it permits undulatory move- that unite reptiles to the exclusion of niata. The vertebrates (craniates) may ments of the body. In most protochor- birds and mammals. Similarly, agnathans be variously subdivided into groups dates and in jawless vertebrates, the (hagfishes and lampreys) are a para- based on shared possession of charac- notochord persists throughout life phyletic grouping because the most teristics. Two such subdivisions shown (Figure 25-1). In all vertebrates a recent common ancestor of agnathans is in Table 25-1 are: (1) Agnatha, verte- series of cartilaginous or bony verte- also an ancestor of all remaining verte- brates lacking jaws (hagfishes and lam- brae are formed from mesenchymal brates (the gnathostomes). The reasons preys), and Gnathostomata, vertebrates cells derived from blocks of meso- why paraphyletic groups are not used in having jaws (all other vertebrates) and dermal cells (somites) lateral to the cladistic taxonomy are explained in (2) Amniota, vertebrates whose embryos notochord. In most vertebrates, the Chapter 10 (p. 201). develop within a fluid-filled sac, the notochord is entirely displaced by ver- The phylogenetic tree of the chor- amnion (reptiles, birds, and mammals), tebrae, although remains of the noto- dates (Figure 25-2) and the cladogram and Anamniota, vertebrates lacking chord usually persist between or of the chordates (Figure 25-3) provide this adaptation (fishes and amphib- within the vertebrae.

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CHAPTER 25 Chordates 491

Mammals Birds

Modern reptiles

Modern amphibians

Modern bony fishes

Primitive amniotes

Early tetrapods Cartilaginous fishes

Placoderms Jawless fishes

Lancelet

Ostracoderms

ta Tunicates orda phal och Ce Urochordata Free-swimming chordate ancestor

Echinodermata Echinoderms Primitive sessile filter-feeder

Hemichordates Hypothetical deuterostomian ancestor

CENOZOIC PRECAMBRIAN PALEOZOIC MESOZOIC TO PRESENT

Geologic time (My ago) 570 225 65

Figure 25-2 Phylogenetic tree of the chordates, suggesting probable origin and relationships. Other schemes have been suggested and are possible. The relative abundance in numbers of species of each group through geological time, as indicated by the fossil record, is suggested by the bulging and thinning of that group’s line of descent.

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492 PART 3 The Diversity of Animal Life

Chordata (animals with notochord at some stage in life cycle) Euchordata (chordates that retain an axial skeleton throughout life) Craniata (euchordates with a cranium) = Vertebrata Gnathostomata (Craniata with jaws) Teleostomi (bony fishes + tetrapods) Tetrapoda (four-legged vertebrates) Amniota (tetrapods with embryos having extraembryonic membranes) Osteichthyes Protochordata Agnatha Reptilia (bony fishes)

Actinopterygii Cephalochordata Cephalaspidomorphi (ray-finned fishes) Amphibia Lepidosauria Aves (lancelets) (lampreys) Sarcopterygii (lizards, snakes) (birds) (lungfishes, lobe Urochordata Myxini -finned fishes) Testudines Mammalia (tunicates) (hagfishes) (turtles) Crocodilia (mammals) Chondrichthyes (sharks, rays, chimaeras)

Hair, mammary glands Naked skin, Body fusiform, suckerlike oral heterocercal disc, long larval caudal fin, stage, 7 pairs placoid scales, Egg with extraembryonic membranes of gills cartilaginous skeleton Paired limbs used for terrestrial locomotion Naked skin Unique supportive elements in skeleton or with slime girdles of fins or legs glands, degenerate Lung or swimbladder derived from gut, bony skeleton eyes, notochord Jaws, 3 pairs semicircular canals, paired appendages, Notochord and persistent nerve cord in free- gill filaments lateral to gill support swimming larvae 2 or 3 pairs semicircular canals, mesonephric kidney only Distinct head and brain; specialized sense organs; 1 or more pairs semicircular canals, pronephric kidney, neural crest, neurogenic epidermal placodes Axial skeleton retained throughout life; muscle somites present

Notochord; dorsal hollow nerve cord; pharyngeal slits; postanal tail

Figure 25-3 Cladogram of living members of phylum Chordata showing probable relationships of monophyletic groups composing the phylum. Each branch in the cladogram represents a monophyletic group. Some derived character states that identify the branchings are shown at right of the branch points. Nesting brackets across the top of the cladogram identify monophyletic groupings within the phylum. The term Craniata, although commonly equated with Vertebrata, is preferred by many authorities because it recognizes that jawless vertebrates (Agnatha) have a cranium but no vertebrae. The lower set of brackets identify the traditional groupings Protochordata, Agnatha, Osteichthyes, and Reptilia. These paraphyletic groups are not recognized in cladistic treatments, but are shown because of widespread use.

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CHAPTER 25 Chordates 493

TABLE 25.1 Traditional Divisions of the Phylum Chordata

Urochordata Cephalo- Myxini Cephala- Chondrich- Osteichthyes Amphibia Reptilia Aves Mammalia (tunicates) chordata (hagfishes) spidomorphi thyes (bony fishes) (amphibians) (reptiles) (birds) (mammals) (lancelets) (lampreys) (sharks)      

Brain Dorsal nerve cord and the evagination, or outpocketing, Segmented myotomes between septa anchored to notochord of the endodermal lining of the phar- ynx (pharyngeal pouches). In aquatic chordates, the two pockets break through the pharyngeal cavity where

they meet to form the pharyngeal slit. Postanal tail Dorsal Tubular Nerve Cord In amniotes these pockets may not In most invertebrate phyla that have a break through the pharyngeal cavity nerve cord, it is ventral to the alimen- and only grooves are formed instead Postanal Tail tary canal and is solid, but in chordates of slits. In tetrapod (four-footed) verte- The postanal tail, together with so- the single cord is dorsal to the alimen- brates the pharyngeal pouches give matic musculature and the stiffening tary canal and is a tube (although the rise to several different structures, notochord, provides the motility that hollow center may be nearly obliter- including the Eustachian tube, middle larval tunicates and amphioxus need ated during growth). The anterior end ear cavity, tonsils, and parathyroid for their free-swimming existence. As a becomes enlarged to form the brain. glands (see pp. 175–176). structure added to the body behind the The hollow cord is produced in the The perforated pharynx evolved as end of the digestive tract, it clearly has embryo by the infolding of ectodermal a filter-feeding apparatus and is used evolved specifically for propulsion in cells on the dorsal side of the body as such in the protochordates. Water water. Its efficiency is later increased in above the notochord. Among the ver- with suspended food particles is drawn fishes with the addition of fins. The tail tebrates, the nerve cord passes through by ciliary action through the mouth is evident in humans only as a vestige the protective neural arches of the ver- and flows out through the pharyngeal (the coccyx, a series of small vertebrae tebrae, and the anterior brain is sur- slits where food is trapped in mucus. at the end of the spinal column) but rounded by a bony or cartilaginous Later, in vertebrates, ciliary action most other mammals have a waggable cranium. was replaced by a muscular pump that tail as adults. drives water through the pharynx by expanding and contracting the pharyn- Pharyngeal slits between aortic arches Ancestry and geal cavity. Also modified were the aortic arches that carry blood through Evolution the pharyngeal bars. In protochordates these are simple vessels surrounded by Since the mid-nineteenth century when connective tissue. The early fishes the theory of organic evolution became added a capillary network having only the focal point for ferreting out rela- tionships among groups of living Pharyngeal Pouches thin, gas-permeable walls, thus improv- ing efficiency of gas transfer between organisms, zoologists have debated the and Slits blood and the water outside. These question of chordate origins. It has Pharyngeal slits are perforated slitlike adaptations led to the evolution of been very difficult to reconstruct lines openings that lead from the pharyn- internal gills, completing the conver- of descent because the earliest proto- geal cavity to the outside. They are sion of the pharynx from a filter-feed- chordates were in all probability soft- formed by the inpocketing of the out- ing apparatus in protochordates to a bodied creatures that stood little side ectoderm (pharyngeal grooves) respiratory organ in aquatic vertebrates. chance of being preserved as fossils

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494 PART 3 The Diversity of Animal Life

Pharyngeal slits Mouth and phosphate). Thus, while we do Most of the early efforts to identify kinship not yet understand the precise struc- of chordates to other phyla are now recog- ture of the long-sought chordate ances- nized as based on similarities related to tor, we do know two living protochor- analogy rather than homology. Analogous structures are those that perform similar date groups that descended from it. functions but have altogether different ori- These we will now consider. gins (such as wings of birds and butterflies) Homologous structures, on the other hand, share a common origin but may look differ- Subphylum ent (at least superficially) and perform quite different functions. For example, all Urochordata vertebrate forelimbs are homologous because they are derived from a pen- (Tunicata) tadactyl limb of the same ancestor, even The urochordates (―tail-chordates‖), though they may be modified as differently more commonly called tunicates, as the human arm and a bird’s wing. Homol- include about 3000 species. They are ogous structures share a genetic heritage; Anus Tail found in all seas from near shoreline to analogous structures do not. Obviously, only great depths. Most are sessile as adults, homologous similarities have any bearing in Figure 25-4 ancestral connections. Fossil of an early echinoderm, a calcichordate, although some are free living. The that lived during the Ordovician period (450 name ―tunicate‖ is suggested by the million years BP). It shows affinities with both usually tough, nonliving tunic, or test, echinoderms and chordates and may belong to even under the most ideal conditions. a lineage that was ancestral to chordates. that surrounds the animal and contains Consequently, such reconstructions cellulose (Figure 25-5). As adults, tuni- largely come from the study of living cates are highly specialized chordates, organisms, especially from an analysis for in most species only the larval form, of early developmental stages, which period some 570 million years ago, the which resembles a microscopic tad- tend to be more evolutionarily con- first distinctive chordates arose from a pole, bears all the chordate hallmarks. served than the differentiated adult lineage related to echinoderms and During adult metamorphosis, the noto- forms that they become. hemichordates (Figure 25-2; see also chord (which, in the larva, is restricted Zoologists at first speculated that Figure 24-7, p. 486). chordates evolved within the proto- While modern echinoderms look stome lineage (annelids and arthro- nothing at all like modern chordates, Incurrent pods) but discarded such ideas when evolutionary affinity between chor- Nerve siphon ganglion they realized that supposed morpho- dates and echinoderms gains support Excurrent Sensory logical similarities had no developmen- from fossil evidence. One curious siphon tentacles tal basis. Early in this century when group of fossil echinoderms, the Calci- Pigment further theorizing became rooted in chordata, have pharyngeal slits and spots developmental patterns of animals, it possibly other chordate attributes (Fig- Tunic became apparent that the chordates ure 25-4, see also p. 475). These small, Atrium must have originated within the nonsymmetrical forms have a head Pharynx Genital deuterostome branch of the animal resembling a long-toed medieval boot, duct Endostyle kingdom. As explained earlier (p. 162 a series of pharyngeal slits covered and Figure 8-9), the Deuterostomia, a with flaps much like the gill openings Anus Pharyngeal grouping that includes the echino- of sharks, a postanal tail, and struc- slits derms, hemichordates, lophophorates, tures that are doubtfully interpreted as and chordates, has several important notochord and muscle blocks. These Intestine embryological features that clearly sep- creatures apparently used their pha- Mantle arate it from the Protostomia and ryngeal slits for filter feeding, as do establish its monophyly. Thus the protochordates today. Although calci- Heart Tunic deuterostomes are almost certainly a chordates seem to have some of the Gonads natural grouping of interrelated ani- right chordate characters based on soft (ovary and mals that have their common origin in anatomy, there is no convincing simi- Stomach testes) ancient Precambrian seas. Several lines larity between the hard skeleton of cal- of anatomical, developmental, and cichordates (which was calcium car- Stolons molecular evidence suggest that some- bonate) and that of vertebrates (which Figure 25-5 what later, at the base of the Cambrian is composed of a complex of calcium Structure of a common tunicate, Ciona sp.

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CHAPTER 25 Chordates 495 to the tail, hence the group name Uro- Apparently this gland samples the chordata) and the tail disappear alto- water coming into the pharynx and gether, while the dorsal nerve cord may additionally perform an endocrine becomes reduced to a single ganglion. function concerned with reproduction. Urochordata is divided into three A notochord is lacking in adult sea classes: Ascidiacea (Gr. askiolion, lit- squirts. tle bag, acea, suffix), Larvacea (L. Sea squirts are hermaphroditic, larva, ghost, acea, suffix), and with usually a single ovary and a single Thaliacea (Gr. thalia, luxuriance, testis in the same animal. Germ cells acea, suffix). Of these the members of are carried by ducts into the atrial cav- Ascidiacea are by far the most com- ity, and then into the surrounding mon, diverse, and best known. They Figure 25-6 water where fertilization occurs. Compound sea squirt Botryllus sp., common in are often called ―sea squirts‖ because shallow coastal waters and rock tide pools. Of the four chief characteristics of some species forcefully discharge a jet Each of the star-shaped patterns represents a chordates, adult sea squirts have only of water from the excurrent siphon colonial arrangement in which the arms of the one: pharyngeal slits. However, the lar- when irritated. All but a few ascidian star are individual organisms, each with its own val form gives away the secret of their species are sessile animals, attached to incurrent siphon at the end of the arm. All are true relationship. The tadpole larva united centrally where they share a common rocks or other hard substrates such as test, forming a compound tunicate. (Figure 25-7) is an elongate, transpar- pilings or bottoms of ships. In many ent form with all four chordate charac- areas, they are among the most abun- teristics: notochord, hollow dorsal dant of intertidal animals. nerve cord, propulsive postanal tail, Ascidians may be solitary, colonial, trapped on the mucous net, which is and a large pharynx with endostyle or compound. Each of the solitary and then worked into a rope and carried and pharyngeal slits. The larva does colonial forms has its own test, but posteriorly by cilia into the esophagus not feed but swims for some hours among the compound forms many and stomach. Nutrients are absorbed in before fastening itself vertically by its individuals may share the same test the midgut and indigestible wastes are adhesive papillae to a solid object. It (Figure 25-6). In some compound discharged from the anus, located near then undergoes a dramatic metamor- ascidians each member has its own the excurrent siphon. phosis (Figure 25-7) to become a ses- incurrent siphon, but the excurrent The circulatory system consists of sile adult, so modified as to become opening is common to the group. a ventral heart and two large vessels, almost unrecognizable as a chordate. Solitary ascidians (Figure 25-5) are one on either side of the heart; these Tunicates of the class Thaliacea, usually spherical or cylindrical forms. vessels connect to a diffuse system of known as thaliaceans or salps, are Lining the tunic is an inner membrane, smaller vessels and spaces serving barrel- or lemon-shaped pelagic forms the mantle. On the outside are two the pharyngeal basket (where respira- with transparent, gelatinous bodies that, projections: the incurrent siphon, or tory exchange occurs), the digestive despite the considerable size that some oral siphon, which corresponds to the organs, gonads, and other structures. species reach, are nearly invisible in anterior end of the body, and the An odd feature found in no other chor- sunlit surface waters. They occur singly excurrent siphon, or atrial siphon, date is that the heart drives the blood or in colonial chains that may reach sev- that marks the dorsal side. When the first in one direction for a few beats, eral meters in length (Figure 25-8). The sea squirt is expanded, water enters then pauses, reverses its action, and cylindrical thaliacean body is typically the incurrent siphon and passes into a drives the blood in the opposite direc- surrounded by bands of circular mus- capacious ciliated pharynx that is tion for a few beats. Another remark- cle, with incurrent and excurrent minutely subdivided by gill slits to able feature is the presence of strik- siphons at opposite ends. Water form an elaborate basketwork. Water ingly high amounts of rare elements in pumped through the body by muscular passes through the gill slits into an the blood, such as vanadium and nio- contraction (rather than by cilia as in atrial cavity and out through the bium. The vanadium concentration in ascidians) is used for locomotion by a excurrent siphon. the sea squirt Ciona may reach 2 mil- sort of jet propulsion, for respiration, Feeding depends on the forma- lion times its concentration in seawa- and as a source of particulate food that tion of a mucous net that is secreted ter. The function of these rare metals in is filtered on mucous surfaces. Many are by a glandular groove, the endostyle, the blood is a mystery. provided with luminous organs and located along the midventral side of The nervous system is restricted to give a brilliant light at night. Most of the the pharynx. Cilia on gill bars of the a nerve ganglion and plexus of body is hollow, with the viscera form- pharynx pull the mucus into a sheet nerves that lie on the dorsal side of the ing a compact mass on the ventral side. that spreads dorsally across the inner pharynx. Beneath the nerve ganglion is The life histories of thaliaceans are face of the pharynx. Food particles located the subneural gland, con- often complex and are adapted to brought in the incurrent opening are nected by a duct to the pharynx. respond to sudden increases in their

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496 PART 3 The Diversity of Animal Life

Figure 25-8 Notochord Colonial thaliacean. The transparent individuals Nerve cord Tail of this delicate, planktonic species are grouped in a chain. Visible within each individual is an Pharynx orange gonad, an opaque gut, and a long Heart serrated gill bar.

Free-swimming larva

Stomach Esophagus Pharynx Pharyngeal slit Gonads Mouth

Endostyle House

Spiracle (modified Attached, early pharyngeal metamorphosis slits)

Notochord Incurrent Atrium Degenerating Water filters notochord outflow

Trunk Water Tail Feeding inflow Tail filters Late metamorphosis

Figure 25-9 Larvacean adult (left) and as it appears within its transparent house (right ), which is about the size of a walnut. When the feeding filters become clogged with food, the tunicate abandons its house and builds a new one.

reproduce by an alternation of sexual sages through which the water enters and asexual generations. Thaliaceans (Figure 25-9). Particulate food trapped are believed to have evolved from ses- on a feeding filter inside the house is sile ancestors as did the ascidians. drawn into the animal’s mouth through Adult The third tunicate class, the Lar- a strawlike tube. When the filters Figure 25-7 vacea (Appendicularia in some classifi- become clogged with waste, which Metamorphosis of a solitary ascidian from a cations) are curious larva-like pelagic happens about every 4 hours, the lar- free-swimming tadpole larva stage. creatures shaped like a bent tadpole. vacean abandons its house and builds In fact their resemblance to the larval a new house, a process that takes only food supply. The appearance of a phy- stages of other tunicates has given a few minutes. Like the thaliaceans, the toplankton bloom, for example, is met them their class name of Larvacea. larvaceans can quickly build up dense by an explosive population increase They feed by a method unique in the populations when food is abundant. At leading to extremely high density of animal world. Each builds a delicate such times scuba diving among the thaliaceans. Common forms include house, a transparent hollow sphere of houses, which are about the size of Doliolum and Salpa, both of which mucus interlaced with filters and pas- walnuts, is likened to swimming

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CHAPTER 25 Chordates 497

Dorsal Wheel nerve cord Notochord Dorsal fin organ Myotome Rostrum Caudal fin

Oral hood Pharyngeal Pharyngeal Gonad Hepatic Atriopore Intestine Ventral Anus A with tentacles slits bars cecum fin

Figure 25-10 Amphioxus. This interesting bottom-dwelling cephalochordate illustrates the four distinctive chordate characteristics (notochord, dorsal nerve cord, pharyngeal slits, and postanal tail). The vertebrate ancestor is thought to have had a similar body plan. A, Internal structure. B, Living amphioxus in typical position for filter feeding. Note the oral hood with tentacles surrounding the mouth.

B

through a snowstorm! Larvaceans are characteristics of chordates in simple it to the ventral aorta. Lacking both paedomorphic, that is, they are sexu- form. Water enters the mouth, driven erythrocytes and hemoglobin, their ally mature animals that have retained by cilia in the buccal cavity, then blood is thought to transport nutrients the larval body form of their evolution- passes through numerous pharyngeal but play little role in gas exchange. ary ancestors (see the boxed note slits where food is trapped in mucus, The nervous system is centered explaining paedomorphosis on p. 500). which is then moved by cilia into the around a hollow nerve cord lying above intestine. Here the smallest food parti- the notochord. Pairs of spinal nerve cles are separated from the mucus and roots emerge at each trunk myomeric Subphylum passed into the hepatic cecum (liver (muscle) segment. Sense organs are diverticulum) where they are phagocy- simple, unpaired bipolar receptors Cephalochordata tized and digested intracellularly. As in located in various parts of the body. Cephalochordates are the marine tunicates, the filtered water passes first The ―brain‖ is a simple vesicle at the lancelets: slender, laterally compressed, into an atrium, then leaves the body anterior end of the nerve cord. translucent animals about 5 to 7 cm in by an atriopore (equivalent to the Sexes are separate. The sex cells length (Figure 25-10) that inhabit the excurrent siphon of tunicates). are set free in the atrial cavity, then sandy bottoms of coastal waters around The closed circulatory system is pass out the atriopore to the outside the world. Lancelets originally bore the complex for so simple a chordate. The where fertilization occurs. Cleavage is generic name Amphioxus (Gr. amphi, flow pattern is remarkably similar to total (holoblastic) and a gastrula is both ends, oxys, sharp), later surren- that of primitive fishes, although there formed by invagination. The larvae dered by priority to Branchiostoma is no heart. Blood is pumped forward hatch soon after deposition and gradu- (Gr. branchia, gills, stoma, mouth). in the ventral aorta by peristaltic-like ally assume the shape of adults. Amphioxus is still used, however, as a contractions of the vessel wall, then No other chordate shows the basic convenient common name for all of passes upward through branchial arter- diagnostic chordate characteristics as the approximately 25 species in this ies (aortic arches) in the pharyngeal clearly as amphioxus. In addition to diminutive subphylum. Four species of bars to paired dorsal aortas which join the four chordate anatomical hall- amphioxus are found in North Ameri- to become a single dorsal aorta. From marks, amphioxus possesses several can coastal waters. here the blood is distributed to the structural features that suggest the ver- Amphioxus is especially interest- body tissues by microcirculation and tebrate plan. Among these are a ing because it has the four distinctive then is collected in veins, which return hepatic cecum, a diverticulum that

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498 PART 3 The Diversity of Animal Life resembles the vertebrate pancreas in Living Endoskeleton structural strength required for life on secreting digestive enzymes, seg- The endoskeleton of vertebrates, as in land, where mechanical stresses on the mented trunk musculature, and the the echinoderms, is an internal sup- endoskeleton are far greater than they basic circulatory plan of more portive structure and framework for are in water. advanced chordates. As discussed later the body. This internal location is a (p. 500), many zoologists consider departure in animal architecture, since amphioxus a living descendant of an invertebrate skeletons generally enfold Pharyngeal slits ancestor that gave rise to both the the body. Exoskeletons and endo- Dorsal cephalochordates and the vertebrates. skeletons have their own particular aorta Therefore cephalochordates are, in sets of advantages and limitations that cladistic terms, the sister group of the are related to size (see note on p. 647). Gut vertebrates (Fig-ure 25-3). For vertebrates, the living endoskel- eton possesses an overriding advan- tage over the dead exoskeleton of Subphylum arthropods. Growing with the body Heart Vertebrata as it does, the endoskeleton permits Aortic almost unlimited body size with much arches Ventral aorta (Craniata) greater economy of building materials. The third subphylum of the chordates is Some vertebrates have become the the large and diverse Vertebrata. This most massive animals on earth. The en- Pharynx and Efficient monophyletic group shares the basic doskeleton forms an excellent jointed Respiration chordate characteristics with the other scaffolding for muscles and the muscles The perforated pharynx, present as two subphyla, but in addition it demon- in turn protect the skeleton and cush- pharyngeal pouches in all chordates at strates a number of novel homologies ion it from potentially damaging some stage in their life cycle, evolved that the others do not share. The alter- impact. for filter-feeding. In primitive chordates native name of the subphylum, Crani- We should note that vertebrates (such as amphioxus), water with sus- ata, more accurately describes the have not wholly lost the protective pended food particles is drawn through group since all have a cranium (bony or function of a firm external covering. the mouth by ciliary action and flows cartilaginous braincase) whereas, the The skull and thoracic rib cage enclose out through the pharyngeal slits where jawless fishes lack vertebrae. and protect vulnerable organs. Most food is trapped in mucus. As protover- vertebrates are further protected with a tebrates shifted from filter-feeding to a Adaptations that Have tough integument, often bearing non- predatory life habit, the pharynx Guided Vertebrate living structures such as scales, hair, or became modified into a muscular feed- Evolution feathers that may provide insulation as ing apparatus through which water well as physical security. could be pumped by expanding and From the earliest fishes to the mam- The endoskeleton was probably contracting the pharyngeal cavity. Cir- mals, the evolution of the vertebrates composed initially of cartilage that culation to the internal gills was has been guided by the specialized later gave way to bone. Cartilage forms improved by addition of capillary beds basic adaptations of living endoskele- a perfectly suitable endoskeleton for (lacking in protochordates) and devel- ton, pharynx and efficient respiration, aquatic animals. Cartilage is superior to opment of a ventral heart and muscular advanced nervous system, and paired bone for fast growth and is therefore aortic arches. All of these changes sup- limbs. ideal for constructing the first skeletal ported an increased metabolic rate that framework of all vertebrate embryos. would have to accompany the switch In agnathans (hagfish and lampreys), Centrum of to an active life of selective predation. vertebra sharks and their kin, and even in some Notochord Neural spine bony fishes such as sturgeons, the adult endoskeleton is composed mostly New Head and Advanced or entirely of cartilage. Bone appears in Nervous System the endoskeleton of more derived ver- No single system in the body is more tebrates, perhaps because it offers two strongly associated with functional and clear advantages to cartilage. First, it structural advancement than is the ner- serves as a reservoir for phosphate, an vous system. When vertebrate ances- indispensable component of com- tors shifted from filter feeding to active pounds with high-energy bonds, of predation, new sensory, motor, and membranes, and of nucleic acids. Sec- integrative controls became essential ond, only bone could provide the for location and capture of larger prey

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CHAPTER 25 Chordates 499

Characteristics of and muscular aortic arches; in tetra- vided with ducts to drain the waste Subphylum Vertebrata pods the much reduced pharynx is to cloaca or anal region embryonic source of glandular tissue 10. Highly differentiated brain; 10 or 12 1. Chief diagnostic features of 5. Many muscles attached to the pairs of cranial nerves with both chordates—notochord, dorsal skeleton to provide for movement motor and sensory functions usually; nerve cord, pharyngeal pouches, 6. Complete digestive system ventral to a pair of spinal nerves for each prim- and postanal tail—all present at the spinal column and provided with itive myotome; an autonomic ner- some stage of the life cycle large digestive glands, liver, and pan- vous system in control of involun- 2. Integument basically of two divi- creas tary functions of internal organs; sions, an outer epidermis of strati- 7. Circulatory system consisting of a paired special sense organs fied epithelium from ectoderm and ventral heart of two to four cham- derived from epidermal placodes an inner dermis of connective tissue bers; closed blood vessel system of 11. Endocrine system of ductless derived from mesoderm; many modi- arteries, veins, and capillaries; blood glands scattered through the body fications of skin among the various fluid containing red blood corpuscles 12. Nearly always separate sexes; each classes, such as glands, scales, feath- with hemoglobin and white corpus- sex containing paired gonads with ers, claws, horns, and hair cles; paired aortic arches connecting ducts that discharge their products 3. Distinctive endoskeleton consisting ventral and dorsal aortas and giving either into the cloaca or into special of vertebral column (notochord per- off branches to the gills among gill- openings near the anus sistent in jawless fishes which lack breathing vertebrates; in terrestrial 13. Body plan consisting typically of vertebrae), limb girdles, and two types modification of the aortic arch head, trunk, and postanal tail; pairs of jointed appendages derived plan into pulmonary and systemic neck present in some, especially from somatic mesoderm, and a head systems terrestrial forms; two pairs of append- skeleton (cranium and pharyngeal 8. Well-developed coelom largely filled ages usually, although entirely skeleton) derived largely from neu- with the visceral systems absent in some; coelom divided ral crest cells 9. Excretory system consisting of into a pericardial space and a gen- 4. Muscular, perforated pharynx; in paired kidneys (mesonephric or eral body cavity; mammals with a fishes pharyngeal slits possess gills metanephric types in adults) pro- thoracic cavity

Auditory Neural arch odes. The neural crest, a population of later became prominently developed capsule ectodermal cells lying along the length into legs for locomotion on land. Joint- Eye Spinal of the embryonic neural tube, con- ed limbs are especially suited for life on Brain cord Olfactory tributes to the formation of many differ- land because they permit finely graded bulb ent structures, among them the cra- leveling motions against a substrate. nium, cranial nerves, branchial skeleton, and the aortic arches. The epidermal The Search for the placodes are plate-like ectodermal thickenings (the term ―placode‖ derives Vertebrate Ancestral Stock from a Greek word meaning ―plate‖) The earliest vertebrate Paleozoic fossils, that appear anteriorly on either side of the jawless ostracoderm fishes we con- items. Paired special sense organs the neural tube. These give rise to the sider at the end of this chapter, share designed for distance reception evolved. nose, eyes, ears, taste receptors, and many novel features or organ system These included paired eyes with lenses lateral line mechanoreceptors and elec- development with living vertebrates. and inverted retinas; pressure recep- troreceptors. Thus the vertebrate head These organ systems therefore must tors, such as paired ears designed for with its sensory structures located adja- have originated in either an early verte- equilibrium and later redesigned to cent to the mouth (later equipped with brate or invertebrate chordate lineage. include sound reception; electrorecep- prey-capturing jaws), stemmed from the With one exception, hardly any inverte- tors that could signal the direction of creation of completely new cell types— brate chordates are known as fossils. potential prey; and chemical receptors, a rare event in animal evolution. The exception is Pikaia gracilens, a rib- including taste receptors and exqui- bon-shaped, somewhat fishlike, crea- sitely sensitive olfactory organs. ture about 5 cm in length discovered in Development of the vertebrate Paired Limbs the famous Burgess Shale of British head and paired sense organs was Pectoral and pelvic appendages are Columbia (Figure 25-11). Pikaia is a largely the result of two embryonic present in most vertebrates in the form mid-Cambrian form that precedes the innovations present only in vertebrates: of paired fins or jointed legs. These earliest vertebrate fossils by many mil- the neural crest and epidermal plac- originated as swimming stabilizers and lions of years. This fossil possessed

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500 PART 3 The Diversity of Animal Life

Notochord

Ostracoderm

Tadpole larva Segmented muscle ? METAMORPHOSIS Figure 25-11 Pikaia, the earliest known chordate, from the Adult Burgess Shale of British Columbia, Canada. ascidian

Paedomorphic vertebrate ancestor both a notochord and characteristic Figure 25-12 chordate -shaped muscle bands Garstang’s hypothesis of larval evolution. Adult tunicates live on the sea floor but produce a free- (myotomes). Without question Pikaia is swimming tadpole larva. More than 500 million years ago, some larvae began to reproduce in the a chordate. It shows a remarkable swimming stage. These evolved into ostracoderms, the first known vertebrates. resemblance to living amphioxus, at least in overall body organization, and ate, sessile descendants of the free- the closest living relative of verte- may in fact be an early cephalochor- swimming form. In 1928, Walter brates. No other protochordate shows date. Pikaia, and a slightly older similar Garstang in England introduced fresh the basic diagnostic characteristics of fossil recently discovered in China, thinking into the vertebrate ancestor the chordates so well. However, as called Yunnanozoon, are provocative debate by turning this sequence around; pointed out in the prologue to this fossils but, until other Cambrian chor- rather than the ancestral tadpole larva chapter (p. 488), amphioxus is no date fossils are discovered their relation- giving rise to a degenerative tunicate longer considered a direct ancestor of ship to earliest vertebrates remains sessile adult, he suggested that the ses- the vertebrates, although it may closely uncertain. In the absence of additional sile adults were the ancestral stock. The resemble an ancestor of the vertebrate fossil evidence, most speculations on tadpole larva then evolved as an adapta- lineage. It lacks a brain and all of the vertebrate ancestry have focused on the tion for spreading to new habitats. Next, specialized sensory equipment that living cephalochordates and tunicates, Garstang suggested that at some point characterizes vertebrates. There are no since it is widely believed that verte- the tadpole larva failed to metamor- gills in the pharynx and no mouth or brates must have emerged from a lin- phose into an adult, but developed pharyngeal musculature for pumping eage resembling one of these proto- gonads and reproduced in the larval water through the gill slits; movement chordate groups. stage. With continued larval evolution, a of water is entirely by the action of cilia.

new group of free-swimming animals Garstang’s Hypothesis of appeared (Figure 25-12). Chordate Larval Evolution Garstang called this process paedo- Paedomorphosis, the displacement of morphosis (Gr. pais, child morphƒ, ancestral larval or juvenile features into a At first glance, tunicates seem unlikely form), a term describing the evolu- descendant adult, can be produced by three candidates as ancestors for vertebrates. tionary retention of juvenile or larval different evolutionary-development processes: neoteny, progenesis, and post- The adult tunicate, which spends it life traits in the adult body. Garstang anchored to some marine surface, displacement. In neoteny, the growth rate departed from previous thinking by of body form is slowed so that the animal lacks a notochord, tubular nerve cord, suggesting that evolution may occur postanal tail, sense organs, and seg- does not attain the ancestral adult form at in larval stages of animals—and in this the time it reaches reproductive maturity. mented musculature. Its larva, how- case, lead to the vertebrate lineage. Progenesis is the precocious maturation of ever, bears all the right qualifications Paedomorphosis is a well-known phe- gonads in a larval (or juvenile) body that for chordate membership. Called ―tad- nomenon in several different animal then stops growing and never attains the pole‖ larva because of its superficial groups (paedomorphosis in amphib- adult body form. In post-displacement, the resemblance to larval frogs, this tiny, ians is described on p. 547). Further- onset of a developmental process is delayed site-seeking form has a notochord, hol- more, Garstang’s hypothesis agrees relative to reproductive maturation, so that the ancestral adult form is not attained at low dorsal nerve cord, pharyngeal slits, with the embryological evidence. and postanal tail, as well as a brain and the time of reproductive maturation. Neo- Nevertheless, it remains untested and teny, progenesis and post-displacement sense organs. speculative. At the time of its discovery in 1869, thus describe different ways in which paedomorphosis can happen. Biologists the tadpole larva was considered a Position of Amphioxus use the inclusive term paedomorphosis to descendant of an ancient free-swimming describe the results of these evolutionary- chordate ancestor of tunicates. The For many years zoologists believed developmental processes. adults then were regarded as degener- that the cephalochordate amphioxus is

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CHAPTER 25 Chordates 501

Dorsal Nerve aorta Stomach Pronephros cord Myotomes Notochord Otic vesicle Eye Brain Median nostril Oral Pronephric Gallbladder papillae Coelom duct Liver Intestine Heart Oral Anus Pharyngeal hood bar Thyroid Velum gland Figure 25-13 Ammocoete larva, freshwater larval stage of a sea lamprey. Although they resemble amphioxus in many ways, ammocoetes have a well-developed brain, paired eyes, pronephric kidney, and other features lacking in amphioxus but representative of the vertebrate body plan.

Recent studies of the expression of coete larvae are so different from adult slits of amphioxus, there are only homeobox-containing genes which con- lampreys that the mistake is understand- seven pairs of pharyngeal pouches trol the body plan of chordate embryos able; the exact relationship was not and slits in ammocoetes. From pharyn- (homeobox genes are described on explained until metamorphosis into the geal bars separating the pharyngeal p. 169) suggest that the ancestor of both adult lamprey was observed. slits project gill filaments bearing sec- amphioxus and vertebrates was cephal- Ammocoete larvae have a long, ondary lamellae much like the more ized; it had a head region with a brain slender body with an oral hood sur- extensive gills of modern fishes (see and sense organs. In amphioxus and rounding the mouth much like amphi- Figure 26-28, p. 527). Ammocoetes other cephalochordates the notochord oxus (Figure 25-13). Ammocoetes are also have a true liver replacing the grows forward to the anterior tip of the filter feeders, but instead of drawing hepatic cecum of amphioxus, a gall- animal, obliterating most traces of the water by ciliary action into the pharynx bladder, and pancreatic tissue (but no primitive head region. Despite these as amphioxus does, ammocoetes pro- distinct pancreatic gland). specializations and others peculiar to duce a feeding current by muscular Ammocoete larvae display the modern cephalochordates, many zoolo- pumping action much like modern most primitive condition for these char- gists believe that amphioxus has largely fishes. In the floor of the pharynx is an acteristics of any living vertebrate. It retained the primitive pattern of the endostyle, as in amphioxus. The clearly illustrates many shared derived immediate prevertebrate condition. Thus endostyle produces a food-ensnaring characters of vertebrates that are cephalochordates are probably the sister mucus that is passed directly to the obscured in the development of other group of vertebrates (Figure 25-3). intestine. The arrangement of body vertebrates. It may approach most muscle into myotomes, the presence of closely the supposed body plan of the The Ammocoete Larva of a notochord serving as chief skeletal ancestral vertebrate. Lampreys as a Model of axis, and the plan of the circulatory the Primitive Vertebrate system all closely resemble these fea- The Earliest Vertebrates: tures in amphioxus. Body Plan Ammocoetes do have several char- Jawless Ostracoderms Lampreys (jawless fishes of the class acteristics lacking in amphioxus that are The earliest vertebrate fossils are late Cephalaspidomorphi, discussed in the homologous to those of vertebrates. Cambrian articulated skeletons from next chapter) have a freshwater larval These include a two-chambered heart the United States, Bolivia, and Aus- stage known as the ammocoete (atrium and ventricle), a three-part tralia. They were small, jawless crea- (Figure 25-13). In body form, appear- brain (forebrain, midbrain, hindbrain), tures collectively called ostracoderms ance, life habit, and most anatomical special sense organs derived from (os-trak o-derm) (Gr. ostrakon, shell, details, the ammocoete larva resembles epidermal placodes (two eyes, one on derma, skin), which belong to the amphioxus. In fact, lamprey larvae were each side of the midbrain; a median Agnatha division of the vertebrates. given the genus name Ammocoetes (Gr. nostril; and auditory vesicles located These earliest ostracoderms lacked ammos, sand, koitƒ, bed, referring to lateral to the midbrain), a thyroid paired fins that later fishes found so the preferred larval habitat) in the nine- gland, and a pituitary gland. The kid- important for stability (Figure 25-14). teenth century when it was erroneously ney is pronephric (p. 670) and con- The swimming movements of one of thought to be an adult cephalochordate, forms to the basic vertebrate plan. the early groups, the heterostracans closely allied with amphioxus. Ammo- Instead of the numerous pharyngeal (Gr. heteros, different, ostrakon,

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502 PART 3 The Diversity of Animal Life

Figure 25-14 Three ostracoderms, jawless fishes of Silurian and Devonian times. They are shown as they might have appeared while searching for food on the floor of a Devonian sea. All were probably filter-feeders, but employed a strong pharyngeal pump to circulate water rather than the much more limiting mode of ciliary feeding used by their protovertebrate ancestors (presumably resembling amphioxus for this feature). Modern lampreys are believed to be derived from the anaspid group. shell) (also called pteraspidiforms), Coexisting with heterostracans was an advance of great significance must have been clumsy, although suffi- throughout much of the Devonian in vertebrate history. As a group the cient to propel them along the ocean period were osteostracans (Gr. bottom-feeding ostracoderms enjoyed bottom where they searched for food. osteon, bone, ostrakon, shell) (also a respectable radiation in the Silurian With fixed circular or slitlike mouth called cephalaspidiforms). Osteostra- and Devonian periods. openings they probably filtered small cans improved the efficiency of their food particles from the water or ocean benthic life by evolving paired pectoral The Swedish paleozoologist Erik Stensiö bottom. However, unlike the ciliary fins that provided control over pitch was the first to approach fossil anatomy filter-feeding protochordates, ostraco- and yaw. This innovation ensured with the same painstaking attention to derms sucked water into the pharynx well-directed forward movement. A minute detail that morphologists have long by muscular pumping, an important typical osteostracan, such as Cepha- applied to the anatomical study of living innovation that suggests to some laspis (Gr. kephalƒ, head, aspis, fishes. He developed novel and exacting authorities that ostracoderms may have shield) (Figure 25-14), was a small ani- methods for gradually grinding away a fos- been mobile predators that fed on soft- mal, seldom exceeding 30 cm in sil, a few micrometers at a time, to reveal bodied animals. length. It was covered by a well- internal features. He was able to recon- developed armor—the head by a solid struct not only bone anatomy, but nerves, blood vessels, and muscles in numerous The term ―ostracoderm‖ does not denote a shield and the body by bony plates— groups of Paleozoic and early Mesozoic natural evolutionary assemblage but rather but it had no axial skeleton or verte- fishes. His innovative methods are widely is a term of convenience for describing sev- brae. Their jawless mouth was tooth- used today by paleozoologists. eral groups of heavily armored extinct jaw- less. Other distinctive features included less fishes. a sensory lateral line system, paired eyes with complex eye muscle pat- For decades, geologists have used During the Devonian period, the terns, and inner ears with semicircular strange microscopic, toothlike fossils heterostracans underwent a major radi- canals. called conodonts (Gr. k|nos, cone, ation, resulting in the appearance of Another group of ostracoderms, odontos, tooth) to date Paleozoic several peculiar-looking forms varying the anaspids, (Figure 25-14) were marine sediments without having any in shape and length of the snout, dor- more streamlined and more closely idea what kind of creature originally sal spines, and dermal plates. Without resembled modern-day jawless fishes possessed these elements. The dis- ever evolving paired fins or jaws, these (lamprey, for example) than any covery in the early 1980s of fossils earliest vertebrates flourished for 150 other ostracoderm. The evolution of of complete conodont animals has million years until becoming extinct the basic vertebrate head pattern in changed this situation: conodont ele- near the end of the Devonian period. ostracoderms, although lacking jaws, ments belonged to a small early marine

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CHAPTER 25 Chordates 503 vertebrate (Figure 25-15). It is widely presence of jaws is a derived character plates that could be manipulated some- believed that as more is learned about state shared by all jawed fishes and what like jaws with the gill arch muscu- conodont animals they will play an tetrapods. Agnathans, however, are lature. Later, the anterior gill arches important role in understanding the defined principally by the absence of a became hinged and bent forward into origin of vertebrates. At present, how- feature—jaws—that characterize the the characteristic position of vertebrate ever, their position in vertebrate phy- gnathostomes. Therefore the super- jaws. Evidence for this remarkable logeny is a matter of debate. class Agnatha may be paraphyletic. transformation is threefold. First, both The origin of jaws was one of the gill arches and jaws form from upper most important events in vertebrate and lower bars that bend forward and Early Jawed Vertebrates evolution. The utility of jaws is obvious: are hinged in the middle (Figure 25-16). All jawed vertebrates, whether extinct they allow predation on large and Second, both gill arches and jaws are or living, are collectively called active forms of food not available to derived from neural crest cells rather gnathostomes (―jaw mouth‖) in con- jawless vertebrates. Ample evidence than from mesodermal tissue, the trast to the jawless vertebrates, the suggests that jaws arose through modi- source of most bones. Third, the jaw agnathans (―without jaw‖). Living fications of the first two of the serially musculature is homologous to the orig- agnathans, the naked hagfishes and repeated cartilaginous gill arches. We inal gill support musculature. Nearly as lampreys, also are often called cyclo- can see the beginnings of this trend in remarkable as this drastic morphologi- stomes (―circle mouth‖). The gnatho- some ostracoderms where the mouth cal remodeling is the subsequent evolu- stomes are a monophyletic group since becomes bordered by strong dermal tionary fate of jawbone elements—their

Figure 25-15 Restoration of a living conodont animal. Conodonts superficially resembled amphioxus, but they possessed a much greater degree of Notochord encephalization (large, paired eyes, possible Myomeres auditory capsules) and bonelike mineralized elements—all indicating that conodont animals were vertebrates. Conodont elements are believed to be gill-supporting structures or part of a filter-feeding apparatus. Eye Conodont elements

Figure 25-16 Hyomandibular cartilage How vertebrates got their jaw. The resemblance Palatoquadrate cartilage between jaws and the gill supports of the Braincase primitive fishes such as this carboniferous shark suggests that the upper jaw (palatoquadrate) and lower jaw (Meckel’s cartilage) evolved from structures that originally functioned as gill supports. The gill supports immediately behind the jaws are hinged like jaws and served to link the jaws to the braincase. Relics of this transformation are seen during the development of modern sharks.

Meckel's cartilage

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504 PART 3 The Diversity of Animal Life

Figure 25-17 Early jawed fishes of the Devonian period, 400 million years ago. Shown are a placoderm (left) and a related acanthodian (right). Jaws and the gill supports from which the jaws evolved develop from neural crest cells, a diagnostic character of vertebrates. Most placoderms were bottom dwellers that fed on detritus although some were active predators. Acanthodians, the earliest-known true jawed fishes, carried less armor than placoderms. Most were marine but several species entered fresh water.

Traditional Linnean (Probably a paraphyletic ichthys, a fish): bony fishes. Classification of Living group.) Primitively fusiform body but Class Myxini (mik-sin y) variously modified; mostly ossi- Members of Phylum (Gr. myxa, slime): hag- fied skeleton; single gill open- Chordata fishes. Terminal mouth with ing on each side covered with Phylum Chordata four pairs of tentacles; buccal operculum; usually swim blad- funnel absent; nasal sac with Group Protochordata (Acrania) der or lung. duct to pharynx; 5 to 15 Subphylum Urochordata (u ro- Class Amphibia (am-fib e-a) pairs of pharyngeal pouches; kor-da ta) (Gr. oura, tail, L. (Gr. amphi, both or double, partially hermaphroditic. chorda, cord, ata, characterized bios, life): amphibians. Ecto- Class Cephalaspidomor- by) (Tunicata): tunicates. Noto- thermic tetrapods; respiration phi (sef-a-lass pe-do-morf e) chord and nerve cord in free- by lungs, gills, or skin; develop- (Gr. kephalƒ, head, aspi- swimming larva only; ascidian ment through larval stage; skin dos, shield, morphƒ, form) adults sessile, encased in tunic. moist, containing mucous (Petromyzones): lam- Subphylum Cephalochordata glands, and lacking scales. preys. Suctorial mouth with (sef a-lo-kor-da ta) (Gr. kephalƒ, Class Reptilia (rep-til e-a) (L. horny teeth; nasal sac not head, L. chorda, cord): repere, to creep): reptiles. connected to mouth; seven lancelets (amphioxus). Noto- Ectothermic tetrapods possessing pairs of pharyngeal pouches. chord and nerve cord found along lungs; embryo develops within Superclass Gnathostomata entire length of body and persist shelled egg; no larval stage; skin (na tho-sto ma-ta) (Gr. gnathos, throughout life; fishlike in form. dry, lacking mucous glands, and jaw, stoma, mouth): jawed Group Craniata covered by epidermal scales. (A fishes, all tetrapods. With jaws Subphylum Vertebrata (ver te- paraphyletic group.) and (usually) paired appendages. bra ta) (L. vertebratus, backboned). Class Aves (ay veez) (L. pl. of Class Chondrichthyes (kon- Bony or cartilaginous vertebrae avis, bird): birds. Endothermic drik thee-eez) (Gr. chondros, surrounding spinal cord; notochord vertebrates with front limbs cartilage, ichthys, a fish): in embryonic stages, persisting in modified for flight; body covered sharks, skates, rays, chi- some fishes; also may be divided with feathers; scales on feet. maeras. Streamlined body with into two groups (superclasses) Class Mammalia (ma-may lee- heterocercal tail; cartilaginous according to presence of jaws. a) (L. mamma, breast): mam- skeleton; five to seven gills with Superclass Agnatha (ag na- mals. Endothermic vertebrates separate openings, no opercu- tha) (Gr. a, without, gnathos, possessing mammary glands; lum, no swim bladder. jaw) (Cyclostomata): hag- body more or less covered Class Osteichthyes (ost e- fishes, lampreys. Without true with hair; well-developed ik thee-eez) (Gr. osteon, bone, jaws or paired appendages. neocerebrum.

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CHAPTER 25 Chordates 505 transformation into ear ossicles of the Evolution of Modern group, distinguished by a large num- mammalian middle ear (see the note Fishes and Tetrapods ber of shared characteristics. We still on p. 741). do not know, however, from which Reconstruction of the origins of the Among the first jawed vertebrates chordate group the vertebrate lineage vast and varied assemblage of modern were the heavily armored placoderms originated. Early in their evolution, ver- living vertebrates is based, as we have (plak o-derm) (Gr. plax, plate, derma, tebrates divided into agnathans and seen, largely on fossil evidence. Unfor- skin). These first appear in the fossil gnathostomes. These two groups differ record in the early Devonian period tunately the fossil evidence for the ear- from each other in many fundamental (Figure 25-17). Placoderms evolved a liest vertebrates is often incomplete ways, in addition to absence of jaws in great variety of forms, some very large and tells us much less than we would the former group and their presence in (one was 10 m in length!) and like to know about subsequent trends the latter. The appearance of both jaws grotesque in appearance. They were in evolution. Affinities become much and paired fins were major innovations armored fish covered with diamond- easier to establish as the fossil record in vertebrate evolution, among the shaped scales or with large plates of improves. For instance, the descent of most important reasons for the subse- bone. All became extinct by the end of birds and mammals from early tetra- quent major radiations of vertebrates the Paleozoic era and appear to have pod ancestors has been worked out in that produced the modern fishes and left no descendants. However, the a highly convincing manner from the all of the tetrapods, including you, the relatively abundant fossil record avail- acanthodians (Figure 25-17), a group reader of this book. able. By contrast, the ancestry of mod- of early jawed fishes that were contem- ern fishes is shrouded in uncertainty. porary with the placoderms, may have Despite the difficulty of clarifying given rise to the great radiation of early lines of descent for vertebrates, bony fishes that dominate the waters they are clearly a natural, monophyletic of the world today.

Summary

Phylum Chordata is named for the rodlike in the Precambrian period, but the true ori- several distinguishing features that permit- notochord that forms a stiffening body axis gin of the chordates is not yet, and may ted the exceptional adaptive radiation of at some stage in the life cycle of every never be, known with certainty. Taken as a the group. Most important of these are the chordate. All chordates share four distinc- whole, chordates have a greater fundamen- living endoskeleton that allows continuous tive hallmarks that set them apart from all tal unity of organ systems and body plan growth and provides a sturdy framework other phyla: notochord, dorsal tubular than have many other phyla. for efficient muscle attachment and action, nerve cord, pharyngeal pouches, and Subphylum Vertebrata includes the a pharynx perforated with slits (lost or postanal tail. Two of the three chordate backboned members of the animal king- greatly modified in higher vertebrates) with subphyla are invertebrates and lack a well- dom (the living jawless vertebrates, the vastly increased respiratory efficiency, developed head. They are the Urochordata hagfishes and lampreys, actually lack verte- advanced nervous system with clear sepa- (tunicates), most of which are sessile as brae but are included with the Vertebrata ration of the brain and spinal cord, and adults but all of which have a free- by tradition because they share numerous paired limbs. swimming larval stage, and the Cephalo- homologies with vertebrates). As a group chordata (lancelets), fishlike forms that vertebrates are characterized by having a include the famous amphioxus. well-developed head, and by their compar-

The chordates may have descended atively large size, high degree of motility, from echinoderm-like ancestors, probably and a distinctive body plan that embodies

Review Questions

1. What characteristics are shared by the vertebrate taxa (refer to Figure 25-3). 4. In debating the question of chordate six deuterostome phyla that indicate a Why are certain traditional groupings origins, zoologists eventually agreed monophyletic group of interrelated such as Reptilia and Agnatha not rec- that the chordates must have evolved animals? ognized in cladistic usage? within the deuterostome assemblage 2. Explain how the use of a cladistic clas- 3. Name four hallmarks shared by all rather than from a protostome group sification for the vertebrates results in chordates, and explain the function of as earlier argued. What embryological important regroupings of the traditional each. evidences support this view? What

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506 PART 3 The Diversity of Animal Life

characteristics does the fossil echino- 7. Both sea squirts (urochordates) and 10. In 1928 Walter Garstang hypothesized derm group Calcichordata possess that lancelets (cephalochordates) are filter- that tunicates resemble the ancestral suggest it might closely resemble the feeding organisms. Describe the filter- stock of the vertebrates. Explain this ancestor of the chordates? feeding apparatus of a sea squirt and hypothesis. 5. Offer a description of an adult tunicate explain in what ways its mode of feed- 11. Distinguish between ostracoderms and that would identify it as a chordate, ing is similar to, and different from, placoderms. What important evolu- yet distinguish it from any other chor- that of amphioxus. tionary advances did each contribute date group. 8. Explain why it is necessary to know to vertebrate evolution? What are con- 6. Amphioxus long has been of interest the life history of a tunicate to under- odonts? to zoologists searching for a vertebrate stand why tunicates are chordates. 12. Explain how we think the vertebrate ancestor. Explain why amphioxus 9. List four adaptations that guided verte- jaw evolved. captured such interest and why it no brate evolution, and explain how each longer is considered to resemble closely has contributed to the success of ver- the direct ancestor of the vertebrates. tebrates.

Selected References

Alldredge, A. 1976. Appendicularians. Sci. Am. Biol. Rev. 64:221–268. Reviews the diag- Long, J. A. 1995. The rise of fishes: 500 million 235:94–102 (July). Describes the biology of nostic characters of protochordates and years of evolution. Baltimore, The Johns larvaceans, which build delicate houses for ancestral vertebrates and presents a sce- Hopkins University Press. An authoritative, trapping food. nario for the protochordate-vertebrate tran- liberally illustrated evolutionary history of Bone, Q. 1979. The origin of chordates. Oxford sition. fishes. Biology Readers, No. 18, New York, Gould, S. J. 1989. Wonderful life: the Burgess Maisey, J. G. 1996. Discovering fossil fishes. Oxford University Press. Synthesis of Shale and the nature of history. New York, New York, Henry Holt & Company. Hand- hypotheses and range of disagreements W.W. Norton & Company. In this book somely illustrated chronology of fish evolu- bearing on an unsolved riddle. describing the marvelous Cambrian fossils tion with cladistic analysis of evolutionary Bowler, P. J. 1996. Life’s splendid drama: evolu- of the Burgess Shale, Gould “saves the best relationships. tionary biology and the reconstruction of for last” by inserting an epilogue on Pikaia, Pough, F. H., J. B. Heiser, and W. N. McFarland. life’s ancestry 1860–1940. Chicago, Univer- the first known chordate. 1996. Vertebrate life, ed. 4. Upper Saddle sity of Chicago Press. Thorough and elo- Gould, S. J. (ed.) 1993. The book of life. New River, New Jersey, Prentice Hall. Vertebrate quent exploration of scientific debates over York, W. W. Norton & Company. A sweep- morphology, physiology, ecology, and reconstruction of history of life on earth; ing, handsomely illustrated view of (almost behavior cast in a cladistic framework. chapter 4 treat theories of chordate and entirely) vertebrate life. Stokes, M. D., and N. D. Holland. 1998. The vertebrate origins. Jeffries, R. P. S. 1986. The ancestry of the verte- lancelet. Am. Sci. 86(6):552–560. Describes Carroll, R. L. 1997. Patterns and processes of brates. Cambridge, Cambridge University the historical role of amphioxus in early vertebrate evolution. New York, Cam- Press. Jeffries argues that the Calcichordata hypotheses of vertebrate ancestry and sum- bridge University Press. A comprehensive are the direct ancestors of the vertebrates, a marizes recent molecular data that has analysis of the evolutionary processes that view that most zoologists are not willing to rekindled interest in amphioxus. have influenced large-scale changes in ver- accept. Still, this book is an excellent sum- tebrate evolution. mary of the deuterostome groups and of the Gans, C. 1989. Stages in the origin of verte- various competing hypotheses of vertebrate brates: analysis by means of scenarios. ancestry.

Zoology Links to the Internet

Visit the textbook’s web site at · Urochordates. Ascidian News. An online newsletter www.mhhe.com/zoology to find live · Vertebrates. focusing on the biology of the urochor- Internet links for each of the references Phylum Chordata, University of Minnesota. dates. It provides links to other ascidian below. sites. Introduction to the Urochordata. University Animal Diversity Web, University of Urochordates. Information on all members of California at Berkeley Museum of Pale- Michigan.Phylum Chordata. General char- ontology site provides photographs and of this phylum, both sessile and pelagic. acteristics of chordates, with the following information on the biology and classifica- links to urochordates and vertebrates. tion of the urochordates.

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C H A P T E R

26

Fishes Phylum Chordata Class Myxini Class Cephalaspidomorphi Class Chondrichthyes Class Actinopterygii Class Sarcopterygii

Hammerhead shark near the Galápagos Islands.

What Is a Fish? in scales of dermal origin. Even this modern concept of the In common (and especially older) usage, the term fish has term ―fish‖ is used for convenience, not as a taxonomic unit, often been used to describe a mixed assortment of water- because fishes do not compose a monophyletic group. The dwelling animals. We speak of jellyfish, cuttlefish, starfish, common ancestor of the fishes is also an ancestor to the crayfish, and shellfish, knowing full well that when we use land vertebrates, which we exclude from the term ―fish,‖ the word ―fish‖ in such combinations, we are not referring unless we use the term in an exceedingly nontraditional to a true fish. In earlier times, even biologists did not make way. Because fishes live in a habitat that is basically alien to such a distinction. Sixteenth century natural historians classi- humans, people have rarely appreciated the remarkable fied seals, whales, amphibians, crocodiles, even hippopota- diversity of these vertebrates. Nevertheless, whether appre- muses, as well as a host of aquatic invertebrates, as fish. ciated by humans or not, the world’s fishes have enjoyed an Later biologists were more discriminating, eliminating first effusive proliferation that has produced an estimated 24,600 the invertebrates and then the amphibians, reptiles, and living species—more than all other species of vertebrates mammals from the narrowing concept of a fish. Today we combined—with adaptations that have fitted them to almost recognize a fish as an aquatic vertebrate with gills, limbs, if every conceivable aquatic environment. No other animal present, in the form of fins, and usually with a skin covered group threatens their domination of the seas. I

507

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508 PART 3 The Diversity of Animal Life

Position in the Animal mals, arose from one lineage of bony partite brain. Other sensory innova- Kingdom fishes, the sarcopterygians (lobe-finned tions include an inner ear with semi- fishes). The evolution of fishes paralled circular canals, an electrosensory sys- The fishes are a vast array of distantly the appearance of numerous advances in tem, intricate lateral line sensory related gill-breathing aquatic vertebrates vertebrate history. systems, and extrinsic eye muscle. with fins. Fishes are the most ancient 3. The development of jaws with teeth and the most diverse of the monophylet- permitted predation of large and ic subphylum Vertebrata within the phy- Biological Contributions active foods. This gave rise to a lum Chordata, constituting five of the 1. The basic vertebrate body plan was predator-prey arms race that became nine living vertebrate classes and one- established in the common ancestor a major shaping element in vertebrate half of the approximately 48,000 recog- of all vertebrates. Foremost was the evolution through the ages. nized vertebrate species. Although they evolution of cellular bone and the 4. The evolution of paired pectoral are a heterogeneous assemblage, they first endoskeleton. The vertebral and pelvic fins supported by shoul- exhibit phylogenetic continuity within column replaced the notochord as the group and with the tetrapod verte- der and hip girdles provided greatly the main stiffening axis in most adult brates. The jawless fishes, hagfishes and improved maneuverability and vertebrates and provided attachment lampreys, are the living forms that became the precursors of arms and for the skull, many muscles, and the resemble most closely the armored ostra- legs of tetrapod vertebrates. appendages. coderms that appeared in the Cambrian 5. Fishes developed the appropriate 2. With the brain and spinal cord period of the Paleozoic. The living jawed physiological adaptations that enabled enclosed and protected within the fishes, cartilaginous and bony fishes, are them to invade every conceivable cranium and vertebral column, the related phylogenetically to the acantho- type of aquatic habitat. The origin of early fishes were the first animals to dians, a group of jawed fishes that were lungs and air gulping in early lobe- house the central nervous system sep- contemporary with the placoderms of finned fishes permitted limited pene- arate from the rest of the body. Spe- the Silurian and Devonian periods of the tration of semiterrestrial habitats and cialized sense organs for taste, Paleozoic. The tetrapod vertebrates, the prepared for the invasion of land with smell, and hearing evolved with a tri- amphibians, reptiles, birds, and mam- the evolution of tetrapods.

The life of a fish is bound to its body vides a ―distance touch‖ in water. The use of fishes as the plural form of fish form. Their mastery of stream, lake, Thus in mastering the physical prob- may sound odd to most people accustomed and ocean is revealed in the many lems of their element, early fishes to using fish in both the singular and the ways that fishes have harmonized evolved a basic body plan and set of plural. Fish refers to one or more individu- their life design to the physical prop- physiological strategies that both als of the some species; fishes refers to erties of their aquatic surroundings. shaped and constrained the evolution more than one species. Suspended in a medium that is 800 of their descendants. times more dense than air, a trout or The jawless agnathans, the least pike can remain motionless, varying derived of the two groups, include its neutral buoyancy by adding or Ancestry and along with the extinct ostracoderms removing air from the swim bladder. Relationships the living hagfishes and lampreys, Or it may dart forward or at angles, fishes adapted as scavengers or para- using its fins as brakes and tilting rud- of Major Groups sites. Although hagfishes have no ver- ders. With excellent organs for salt of Fishes tebrae and lampreys have only rudi- and water exchange, fishes can mentary vertebrae, they nevertheless steady and finely tune their body The fishes are of ancient ancestry, are included with the subphylum Ver- fluid composition in their chosen having descended from an unknown tebrata because they have a cranium freshwater or seawater environment. free-swimming protochordate ances- and many other vertebrate homolo- Their gills are the most effective res- tor (hypotheses of chordate and verte- gies. The ancestry of hagfishes and piratory devices in the animal king- brate origins are discussed in Chapter lampreys is uncertain; they bear little dom for extracting oxygen from a 25). The earliest fishlike vertebrates resemblance to the extinct ostraco- medium that contains less than 1/20 were a paraphyletic assemblage of derms. Although hagfishes and the as much oxygen as air. Fishes have jawless agnathan fishes, the ostraco- more derived lampreys superficially excellent olfactory and visual senses derms (Figure 25-14, p. 502). One look much alike, they are in fact so dif- and a unique lateral line system, group of the ostracoderms gave rise ferent from each other that they have which with its exquisite sensitivity to to the jawed gnathostomes (Fig- been assigned to separate classes by water currents and vibrations pro- ure 26-1). ichthyologists.

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CHAPTER 26 Fishes 509

Coelacanth

Lungfishes

Modern amphibians

Sturgeons Amniotes

tea ros n o Gars C d ian o ts) h ter (tel e s s yn gian o tery n ne p E Mod a Early amphibians ygia Neo Modern bony fishes

s) sh e hodi f A fi ned i ay s n nchs n - e ygia (r i op Elasm Ac r Sharks, skates, Gnatho- n t rays t stomata

Chondrichthyes Holocephalans Placoderms Chimaeras

Ostracoderms Lampreys Agnatha Common chordate Vertebrata (craniata) ancestor Hagfishes

Cambrian Ordovician Silurian Devonian Carboniferous Permian PALEOZOIC MESOZOIC CENOZOIC

570 225 65 0 Geologic time (My ago)

Figure 26-1 Graphic representation of the family tree of fishes, showing the evolution of major groups through geological time. Numerous lineages of extinct fishes are not shown. Widened areas in the lines of descent indicate periods of adaptive radiation and the relative number of species in each group. The fleshy-finned fishes (sarcopterygians), for example, flourished in the Devonian period, but declined and are today represented by only four surviving genera (lungfishes and coelacanth). Homologies shared by the sarcopterygians and tetrapods suggest that they are sister groups. The sharks and rays radiated during the Carboniferous period. They came dangerously close to extinction during the Permian period but staged a recovery in the Mesozoic era and are a secure group today. Johnny-come-latelies in fish evolution are the spectacularly diverse modern fishes, or teleosts, which make up most living fishes. e-Text Main Menu

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) ns hes fis ed n fin s e- yg lob s( eop ian rl ryg er pte rco Sa ns pter

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obra

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510 PART 3 The Diversity of Animal Life

Craniata = Vertebrata Gnathostomata

Teleostomi Agnatha Chondrichthyes Osteichthyes

Elasmobranchii Actinopterygii Sarcopterygii Myxini Cephalaspidomorphi (sharks, skates, Holocephali (ray-finned (lobe-finned (hagfishes) (lampreys) Placoderms † rays) (chimaeras) Acanthodii † fishes) fishes) Tetrapods Limbs used for Loss of scales, terrestrial "Ostracoderms"† teeth modified as locomotion No paired (anaspids) grinding plates appendages, Unique supportive ele- no dermal bone, ments in skeleton of long larval stage girdle and limbs Lung or swim bladder derived from gut, presence of lepidotrichia (fin rays) Placoid scales, claspers, Gills not attached to interbranchial cartilaginous septum (as they are in sharks), bony skeleton opercular covers

Part of second visceral arch modified as Naked skin with supporting element for jaws, teeth with dentine slime glands, degenerate eyes, Jaws from mandibular arch, 3 pairs semicircular canals, paired accessory hearts appendages with internal skeletal supporting muscles, vertebrae with septum

2 or more pairs semicircular canals

Distinct head, tripartite brain, specialized sense organs, 1 or more pairs semicircular canals, cranium, well-developed visceral skeleton

†Extinct groups Figure 26-2 Cladogram of the fishes, showing the probable relationships of major monophyletic fish taxa. Several alternative relationships have been proposed. Extinct groups are designated by a dagger (†). Some of the shared derived characters that mark the branchings are shown to the right of the branch points. The groups Agnatha and Osteichthyes, although paraphyletic structural grades considered undesirable in cladistic classification, are conveniently recognized in systematics because they share broad structural and functional patterns of organization.

All remaining fishes have paired tinct in the following Carboniferous ous periods of the Paleozoic era but appendages and jaws and are period, leaving no direct descendants. declined dangerously close to extinc- included, along with the tetrapods A second group, the cartilaginous tion at the end of the Paleozoic. They (land vertebrates) in the monophyletic fishes of the class Chondrichthyes staged a recovery in the early Mesozoic lineage of gnathostomes. They appear (sharks, rays, and chimaeras), lost the and radiated to form the modest but in the fossil record in the late Silurian heavy dermal armor of early jawed thoroughly successful assemblage of period with fully formed jaws, and no fishes and adopted cartilage rather modern sharks and rays (Figure 26-1). forms intermediate between agnathans than bone for the skeleton. Most are The other two groups of gnatho- and gnathostomes are known. By the active predators with sharklike or ray- stome fishes, the acanthodians Devonian period, the Age of Fishes, like body forms that have undergone (p. 505) and the bony fishes, were several distinct groups of jawed fishes only minor changes over the ages. As a well represented in the Devonian were well represented. One of these, group, sharks and their kin flourished period. Acanthodians somewhat resem- the placoderms (p. 505), became ex- during the Devonian and Carbonifer- bled bony fishes but were distinguished

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CHAPTER 26 Fishes 511

External gill Mouth opening Pores of Mouth surrounded slime sacs by barbels Teeth on tongue

A B Caudal fin

Spinal chord Pharynx Nostril Olfactory sac Brain Notochord

Barbels

Mouth

C Teeth on Tongue D tongue Internal openings to gill sacs Figure 26-3 The Atlantic hagfish Myxine glutinosa (class Myxini). A, External anatomy; B, Ventral view of head, showing horny plates used to grasp food during feeding; C, Sagittal section of head region (note retracted position of rasping tongue and internal openings into a row of gill sacs); D, Hagfish knotting, showing how it obtains leverage to tear flesh from prey.

by having heavy spines on all fins Superclass Agnatha: ―agnatha‖ as a paraphyletic assemblage except the caudal fin. They became of jawless fishes. extinct in the lower Permian period. Jawless Fishes Although the affinities of the acanthodi- Living jawless fishes are represented ans are much debated, many authors by approximately 84 species divided Class Myxini: Hagfishes believe that they are the sister group of between two classes: Myxini (hag- Hagfishes are an entirely marine the bony fishes. The bony fishes fishes) with about 43 species and group that feeds on annelids, mol- (Osteichthyes, Figure 26-2) are the Cephalaspidomorphi (lampreys) with luscs, crustaceans, and dead or dying domi-nant fishes today. We can recog- 41 species (Figures 26-3 and 26-4). fishes. Thus they are not parasitic like nize two distinct lineages of bony Members of both groups lack jaws, lampreys but are scavengers and fishes. Of these two, by far the most internal ossification, scales, and paired predators. There are 43 described diverse are the ray-finned fishes (class fins, and both groups share porelike species of hagfishes, of which the best Actinopterygii), which radiated to form gill openings and an eel-like body known in North America are the the modern bony fishes. The other lin- form. In other respects, however, the Atlantic hagfish Myxine glutinosa (Gr. eage, the lobe-finned fishes (class Sar- two groups are morphologically very myxa, slime) (Figure 26-3) and the copterygii), although a relic group different. Hagfishes are certainly the Pacific hagfish Eptatretus stouti (N. L. today, carry the distinction of being the least derived of the two, while lam- ept, Gr. hepta, seven tretos, perfo- sister group of the tetrapods. The lobe- preys bear many derived morphologi- rated). Although almost completely finned fishes are represented today by cal characters that place them phyloge- blind, the hagfish is quickly attracted the lungfishes and the coelacanth— netically much closer to gnathostomes to food, especially dead or dying meager remnants of important stocks than to hagfishes. Because of these fishes, by its keenly developed senses that flourished in the Devonian period differences, hagfishes and lampreys of smell and touch. The hagfish enters (Figure 26-1). A classification of the have been assigned to separate verte- a dead or dying animal through an major fish taxa is on p. 534. brate classes, leaving the grouping orifice or by digging inside. Using two

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512 PART 3 The Diversity of Animal Life

Characteristics of Class Myxini

1. Body slender, eel-like, rounded, with naked skin containing slime glands 2. No paired appendages, no dor- sal fin (the caudal fin extends anteriorly along the dorsal surface) 3. Fibrous and cartilaginous skeleton; notochord persistent 4. Biting mouth with two rows of eversible teeth 5. Heart with sinus venosus, atrium, and ventricle; accessory hearts, aortic arches in gill region 6. Five to 16 pairs of gills with a variable number of gill openings 7. Segmented mesonephric kid- ney; marine, body fluids isos- motic with seawater 8. Digestive system without stom- Figure 26-4 ach; no spiral valve or cilia in Sea lamprey, Petromyzon marinus, feeding on the body fluids of a dying fish. intestinal tract 9. Dorsal nerve cord with differenti- ated brain; no cerebellum; 10 pairs of cranial nerves; dorsal and ventral nerve roots united 10. Sense organs of taste, smell, and Hagfishes are renowned for their Class Cephalaspidomorphi hearing; eyes degenerate; one ability to generate enormous quantities pair semicircular canals (Petromyzontes): of slime. If disturbed or roughly han- Lampreys 11. Sexes separate (ovaries and testes dled, the hagfish exudes a milky fluid in same individual but only one is from special glands positioned along All the lampreys of the Northern Hemi- functional); external fertilization; sphere belong to the family Petromy- large yolky eggs, no larval stage the body. On contact with seawater, the fluid forms a slime so slippery that the zontidae (Gr. petros, stone, myzon, animal is almost impossible to grasp. sucking). The group name refers to the Unlike any other vertebrate, the lamprey’s habit of grasping a stone toothed, keratinized plates on the body fluids of hagfishes are in osmotic with its mouth to hold position in a tongue that fold together in a pincer- equilibrium with seawater, as in most current. The destructive marine lam- like action, the hagfish rasps away bits marine invertebrates. Hagfishes have prey Petromyzon marinus is found on of flesh from its prey. For extra lever- several other anatomical and physio- both sides of the Atlantic Ocean (in age, the hagfish often ties a knot in its logical peculiarities, including a low- America and Europe) and may attain a tail, then passes it forward along the pressure circulatory system served by length of 1 m (Figure 26-4). Lampetra body until it is pressed securely three accessory hearts in addition to the (L. lambo, to lick or lap up) also has a against the side of its prey. main heart positioned behind the gills. wide distribution in North America and Eurasia and ranges from 15 to 60 cm The reproductive biology of hag- While the unique anatomical and physiologi- fishes remains largely a mystery, long. There are 22 species of lampreys cal features of the strange hagfishes are of despite a still unclaimed prize offered in North America. About half of these interest to biologists, hagfishes have not more than 100 years ago by the belong to the nonparasitic brook type; endeared themselves to either sports or com- Copenhagen Academy of Science for the others are parasitic. The genus mercial fishermen. In earlier days of commer- information on the animal’s breeding Ichthyomyzon (Gr. ichthyos, fish, cial fishing mainly by gill nets and set lines, habits. It is known that females, which myzon, sucking), which includes three hagfish often bit into the bodies of captured in some species outnumber males 100 parasitic and three nonparasitic species, fish and ate out the contents, leaving behind to one, produce small numbers of sur- is restricted to eastern North America. a useless sack of skin and bones. But as large On the west coast of North America and efficient otter trawls came into use, hag- prisingly large, yolky eggs 2 to 7 cm in the chief marine form is Lampetra fishes ceased to be an important pest. diameter depending on the species. There is no larval stage. tridentatus.

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CHAPTER 26 Fishes 513

Characteristics of Class Cephalaspidomorphi

1. Body slender, eel-like, rounded with naked skin 2. One or two median fins, no paired appendages 3. Fibrous and cartilaginous Parasitic stage in lakes skeleton; notochord persistent Migration towards 4. Suckerlike oral disc and tongue Reproduction lakes in streams with well-developed keratinized teeth 5. Heart with sinus venosus, atrium, and ventricle; aortic arches in gill region 6. Seven pairs of gills each with external gill opening 7. Opisthonephric kidney; anadromous and fresh water; body fluids osmotically and ionically regulated 8. Dorsal nerve cord with differenti- ated brain, small cerebellum Ammocoete present; 10 pairs cranial nerves; Metamorphosis larvae dorsal and ventral nerve roots separated Figure 26-5 9. Digestive system without stom- Life cycle of the “landlocked” form of the sea lamprey Petromyzon marinus. ach; intestine with spiral fold 10. Sense organs of taste, smell, hear- ing; eyes well developed in adult; two pairs semicircular canals The eggs hatch in about 2 weeks, water, where they attach themselves 11. Sexes separate; single gonad releasing small larvae (ammocoetes), by their suckerlike mouth to a fish without duct; external fertilization; long larval stage (ammocoete) which are so unlike their parents that and, with their sharp keratinized teeth, early biologists thought they were rasp away the flesh and suck out body a separate species. The larva bears a fluids (Figure 26-6). To promote remarkable resemblance to amphi- the flow of blood, the lamprey injects All lampreys ascend freshwater oxus and possesses the basic chordate an anticoagulant into the wound. streams to breed. The marine forms are characteristics in such simplified and When gorged, the lamprey releases its anadromous (Gr. anadromos, running easily visualized form that it has been hold but leaves the fish with a large, upward); that is, they leave the sea considered a chordate archetype gaping wound that is sometimes fatal. where they spend their adult lives to (p. 501). After absorbing the remain- The parasitic freshwater adults live swim up streams to spawn. In North der of its yolk supply, the young 1 to 2 years before spawning and then America all lampreys spawn in winter or ammocoete, now about 7 mm long, die; the anadromous forms live 2 to spring. Males begin nest building and leaves the nest gravel and drifts 3 years. are joined later by females. Using their downstream to burrow in some suit- Nonparasitic lampreys do not feed oral discs to lift stones and pebbles and able sandy, low-current area. The after emerging as adults and their ali- vigorous body vibrations to sweep away larva takes up a suspension-feeding mentary canal degenerates to a non- light debris, they form an oval depres- existence while growing slowly for 3 functional strand of tissue. Within a sion (Figure 26-5). At spawning, with to 7 or more years, then rapidly meta- few months they also spawn and die. the female attached to a rock to main- morphoses into an adult. This change The invasion of the Great Lakes by tain her position over the nest, the male involves the eruption of eyes, replace- the landlocked sea lamprey Petromy- attaches to the dorsal side of her head. ment of the hood by the oral disc with zon marinus in this century has had a As eggs are shed into the nest, they are keratinized teeth, enlargement of fins, devastating effect on the fisheries. No fertilized by the male. The sticky eggs maturation of gonads, and modifica- lampreys were present in the Great adhere to pebbles in the nest and tion of the gill openings. Lakes west of Niagara Falls until the quickly become covered with sand. The Parasitic lampreys either migrate to Welland Ship Canal was built in 1829. adults die soon after spawning. the sea, if marine, or remain in fresh Even then nearly 100 years elapsed

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514 PART 3 The Diversity of Animal Life

Horny tongue Characteristics of Class 6. Circulatory system of several pairs Pharynx Chondrichthyes of aortic arches; dorsal and ventral Buccal funnel aorta, capillary and venous systems, with horny 1. Large (average about 2 m), body hepatic portal and renal portal sys- teeth fusiform, or dorsoventrally tems; four-chambered heart with depressed, with a heterocercal sinus venosus, atrium, ventricle, and Lingual cartilage caudal fin (diphycercal in chi- conus arteriosus maeras) (Figure 26-16); paired pec- 7. Respiration by means of five to toral and pelvic fins, two dorsal seven pairs of gills leading to median fins; pelvic fins in male exposed gill slits in elasmobranchs; modified for ―claspers‖ four pairs of gills covered by an 2. Mouth ventral; two olfactory sacs operculum in chimaeras Attachment to fish with horny teeth and suction that do not open into the mouth cav- 8. No swim bladder or lung ity in elasmobranchs; nostrils open 9. Opisthonephric kidney and into mouth cavity in chimaeras; jaws rectal gland; blood isosmotic modified from pharyngeal arch or slightly hyperosmotic to sea 3. Skin with placoid scales or naked water; high concentrations of in elasmobranchs (Figure 26-18); urea and trimethylamine oxide skin naked in chimaeras; teeth of in blood modified placoid scales and serially 10. Brain of two olfactory lobes, replaced in elasmobranchs; teeth two cerebral hemispheres, two modified as grinding plates in chi- optic lobes, cerebellum, medulla maeras oblongata; 10 pairs of cranial 4. Endoskeleton entirely cartilagi- nerves; three pairs of semicircu- nous; notochord persistent but lar canals reduced; vertebrae complete and 11. Senses of smell, vibration reception separate in elasmobranchs; verte- (lateral line system), vision, and Tongue protruded for brae present but centra absent in electroreception well developed; rasping flesh chimaeras; appendicular, girdle, and inner ear opens to outside via Figure 26-6 visceral skeletons present; cranium endolymphatic duct sutureless How the lamprey uses its horny tongue to feed. 12. Sexes separate; gonads paired; After firmly attaching to a fish by its sucker, the 5. Digestive system with J-shaped reproductive ducts open into cloaca protrusible tongue rapidly rasps an opening stomach (stomach absent in chi- (separate urogenital and anal open- through the fish’s integument. Body fluid, maeras); intestine with spiral ings in chimaeras); oviparous, ovovi- abraded skin, and muscle are eaten. valve; often with large oil-filled viparous, or viviparous; direct devel- liver for buoyancy opment; fertilization internal

before sea lampreys were first seen in began to decline after reaching a peak Lake Erie. After that the sea lamprey abundance in 1951 in Lakes Huron and Class spread rapidly and was causing extra- Michigan and in 1961 in Lake Superior. Chondrichthyes: ordinary damage in all the Great Lakes The fall has been attributed both to by the middle 1940s. No fish species depletion of food and to the effective- Cartilaginous Fishes was immune from attack, but the lam- ness of control measures (mainly There are nearly 850 living species in preys preferred lake trout, and this chemical larvicides in selected spawn- the class Chondrichthyes, an ancient, multimillion dollar fishing industry was ing streams). Lake trout, aided by a compact, and highly developed group. brought to total collapse in the late restocking program, are now recover- Although a much smaller and less 1950s. Lampreys then turned to rain- ing. Wounding rates are low in Lake diverse assemblage than the bony bow trout, whitefish, burbot, yellow Michigan but still high in some lakes. fishes, their impressive combination of perch, and lake herring, all important Fishery organizations are now experi- well-developed sense organs, power- commercial species. These stocks were menting with the release of sterilized ful jaws and swimming musculature, decimated in turn. The lampreys then male lampreys into spawning streams; and predaceous habits ensures them a began attacking chubs and suckers. when fertile females mate with steril- secure and lasting place in the aquatic Coincident with decline in attacked ized males the female’s eggs fail to community. One of their distinctive species, sea lampreys themselves develop. features is their cartilaginous skeleton.

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CHAPTER 26 Fishes 515

Thresher shark

Whale shark

Hammerhead shark

Great white shark

Figure 26-7 Diversity in sharks of the subclass Elasmobranchii. The thresher shark Alopias vulpinus, exceptional because of its long upper tail lobe, may exceed 4 m in length. The great white shark Carcharodon carcharias, largest and most notorious of dangerous sharks, is a heavy-bodied, spindle-shaped shark that may reach 6 m in length. The nine species of hammerheads (genus Sphyrna) are distinguished from all other sharks by the flattened head with hammerlike lobes bearing eyes and nostrils on the ends. The whale shark Rhincodon typus is the world’s largest fish, reaching 12 m in length. It is a suspension feeder that feeds on plankton collected on a sievelike mesh over its gills.

Although calcification may be exten- Coastal waters are dominated by the write them off as harmless. It is true, as sive in their skeletons, bone is entirely requiem sharks, order Carcharhini- the latter group of writers argues, that absent throughout the class—a curious formes, which consist of typical-looking sharks are by nature timid and cautious. evolutionary feature, since the Chon- sharks such as the tiger and bull sharks But it also is a fact that certain of them drichthyes are derived from ancestors and more bizarre forms, including the are dangerous to humans. There are having well-developed bone. Almost hammerheads (Figure 26-7). The order numerous authenticated cases of shark all chondrichthyans are marine; only Lamniformes contains several large, attacks by Carcharodon (Gr. karcharos, 28 species live primarily in fresh water. pelagic sharks dangerous to humans, sharp, odous, tooth), the great white With the exception of whales, including the great white and mako shark (reaching 6 m); mako sharks Isu- sharks include the largest living verte- sharks. Dogfish sharks, familiar to gen- rus (Gr. is, equal, ouros, tail); the tiger brates. The larger sharks may reach erations of comparative anatomy stu- shark Galeocerdo (Gr. galeos, shark, 12 m in length. Dogfish sharks so dents, are in the order Squaliformes. kerd|, fox); and hammerhead sharks widely studied in zoological laborato- Skates and several groups of rays (saw- Sphyrna (Gr. sphyra, hammer). More ries rarely exceed 1 m. fish rays, electric rays, stingrays, eagle shark casualties have been reported rays, manta rays, and devil rays) belong from the tropical and temperate waters Subclass Elasmobranchii: to the order Rajiformes. of the Australian region than from Sharks, Skates, and Rays Much has been written about the any other. During World War II there propensities of sharks to attack humans, were several reports of mass shark There are nine living orders of elasmo- both by those exaggerating their fero- attacks on the victims of ship sinkings in branchs, numbering about 815 species. cious nature and by those seeking to tropical waters.

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